Start implementing XEP-0449: Stickers

[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: 68.51% // Head: 68.56% // Increases project coverage by +0.05% :tada:

Coverage data is based on head (fb6ff05) compared to base (9f9d672). Patch coverage: 75.65% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #490 +/- ## ========================================== + Coverage 68.51% 68.56% +0.05% ========================================== Files 294 294 Lines 25340 25788 +448 ========================================== + Hits 17361 17681 +320 - Misses 7979 8107 +128 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490?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/QXmppMessage.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBNZXNzYWdlLmg=) | `100.00% <ø> (ø)` | | | [src/base/QXmppPubSubItem.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBQdWJTdWJJdGVtLmNwcA==) | `100.00% <ø> (ø)` | | | [src/base/QXmppPubSubItem.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBQdWJTdWJJdGVtLmg=) | `75.00% <ø> (ø)` | | | [src/base/QXmppStickerPackItem.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBTdGlja2VyUGFja0l0ZW0uY3Bw) | `71.11% <71.11%> (ø)` | | | [src/base/QXmppMessage.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBNZXNzYWdlLmNwcA==) | `97.98% <83.33%> (-0.33%)` | :arrow_down: | | [src/base/QXmppStickerPackItem.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBTdGlja2VyUGFja0l0ZW0uaA==) | `100.00% <100.00%> (ø)` | | | [tests/qxmppmessage/tst\_qxmppmessage.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBtZXNzYWdlL3RzdF9xeG1wcG1lc3NhZ2UuY3Bw) | `94.89% <100.00%> (+0.06%)` | :arrow_up: | | [tests/qxmpppubsub/tst\_qxmpppubsub.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBwdWJzdWIvdHN0X3F4bXBwcHVic3ViLmNwcA==) | `95.07% <100.00%> (+0.21%)` | :arrow_up: | | [src/base/QXmppFutureUtils\_p.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/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/base/QXmppError.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/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% <0.00%> (ø)` | | | ... and [17 more](https://codecov.io/gh/qxmpp-project/qxmpp/pull/490/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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