File sharing improvements

[lnjX]

• FutureUtils: Add visitForward() function
• Generate QXmppUpload/Download only by FileSharingManager
• FileTransfer: Require some functions to be const
• Make EncryptedHttpFileSharingProvider able to handle not just http
• Rename EncryptedHttpProvider to EncryptedProvider
• HttpFileProvider: Require HttpUploadManager as parameter

PR check list:

• [ ] Document your code
• [ ] Add \since QXmpp 1.X, QXMPP_EXPORT
• [ ] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [ ] Update doc/doap.xml
• [ ] Add unit tests
• [ ] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Base: 68.34% // Head: 68.30% // Decreases project coverage by -0.03% :warning:

Coverage data is based on head (ac52684) compared to base (e1d1b5c). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #488 +/- ## ========================================== - Coverage 68.34% 68.30% -0.04% ========================================== Files 294 292 -2 Lines 25203 25513 +310 ========================================== + Hits 17224 17427 +203 - Misses 7979 8086 +107 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project) | Coverage Δ | | |---|---|---| | [src/base/QXmppError.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBFcnJvci5jcHA=) | `0.00% <0.00%> (ø)` | | | [src/base/QXmppError.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBFcnJvci5o) | `0.00% <ø> (ø)` | | | [src/base/QXmppFileShare.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBGaWxlU2hhcmUuaA==) | `100.00% <ø> (ø)` | | | [src/base/QXmppFutureUtils\_p.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBGdXR1cmVVdGlsc19wLmg=) | `72.41% <0.00%> (-6.84%)` | :arrow_down: | | [src/client/QXmppEncryptedFileSharingProvider.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2NsaWVudC9RWG1wcEVuY3J5cHRlZEZpbGVTaGFyaW5nUHJvdmlkZXIuY3Bw) | `0.00% <0.00%> (ø)` | | | [src/client/QXmppEncryptedFileSharingProvider.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2NsaWVudC9RWG1wcEVuY3J5cHRlZEZpbGVTaGFyaW5nUHJvdmlkZXIuaA==) | `0.00% <0.00%> (ø)` | | | [src/client/QXmppFileSharingManager.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2NsaWVudC9RWG1wcEZpbGVTaGFyaW5nTWFuYWdlci5jcHA=) | `0.00% <0.00%> (ø)` | | | [src/client/QXmppFileSharingManager.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2NsaWVudC9RWG1wcEZpbGVTaGFyaW5nTWFuYWdlci5o) | `0.00% <0.00%> (ø)` | | | [src/client/QXmppFileSharingProvider.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2NsaWVudC9RWG1wcEZpbGVTaGFyaW5nUHJvdmlkZXIuaA==) | `0.00% <0.00%> (ø)` | | | [src/client/QXmppHttpFileSharingProvider.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2NsaWVudC9RWG1wcEh0dHBGaWxlU2hhcmluZ1Byb3ZpZGVyLmNwcA==) | `0.00% <0.00%> (ø)` | | | ... and [5 more](https://codecov.io/gh/qxmpp-project/qxmpp/pull/488/diff?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project)

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