Make encrypted file source public

[jbruechert]

PR check list:

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

[codecov[bot]]

Codecov Report

Base: 68.42% // Head: 68.42% // Decreases project coverage by -0.00% :warning:

Coverage data is based on head (19ad364) compared to base (9f474d2). Patch has no changes to coverable lines.

:exclamation: Current head 19ad364 differs from pull request most recent head 97a6754. Consider uploading reports for the commit 97a6754 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #469 +/- ## ========================================== - Coverage 68.42% 68.42% -0.01% ========================================== Files 279 279 Lines 24306 24305 -1 ========================================== - Hits 16631 16630 -1 Misses 7675 7675 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/469?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/QXmppEncryptedFileSource.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/469/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBFbmNyeXB0ZWRGaWxlU291cmNlLmNwcA==) | `79.01% <ø> (ø)` | | | [src/base/QXmppEncryptedFileSource.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/469/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBFbmNyeXB0ZWRGaWxlU291cmNlLmg=) | `100.00% <ø> (ø)` | | | [tests/qxmppmessage/tst\_qxmppmessage.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/469/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBtZXNzYWdlL3RzdF9xeG1wcG1lc3NhZ2UuY3Bw) | `94.84% <ø> (ø)` | | | [tests/qxmppomemodata/tst\_qxmppomemodata.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/469/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBvbWVtb2RhdGEvdHN0X3F4bXBwb21lbW9kYXRhLmNwcA==) | `98.24% <0.00%> (-0.01%)` | :arrow_down: | 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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