Implement XEP-0294: Jingle RTP Header Extensions Negotiation stanzas

[melvo]

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: 68.34% // Head: 68.51% // Increases project coverage by +0.17% :tada:

Coverage data is based on head (2aa95e5) compared to base (e1d1b5c). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 2aa95e5 differs from pull request most recent head 37fb5d7. Consider uploading reports for the commit 37fb5d7 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #483 +/- ## ========================================== + Coverage 68.34% 68.51% +0.17% ========================================== Files 294 294 Lines 25203 25340 +137 ========================================== + Hits 17224 17361 +137 Misses 7979 7979 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/483?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/QXmppJingleIq.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/483/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBKaW5nbGVJcS5jcHA=) | `89.51% <100.00%> (+0.97%)` | :arrow_up: | | [src/base/QXmppJingleIq.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/483/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBKaW5nbGVJcS5o) | `100.00% <100.00%> (ø)` | | | [tests/qxmppjingleiq/tst\_qxmppjingleiq.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/483/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBqaW5nbGVpcS90c3RfcXhtcHBqaW5nbGVpcS5jcHA=) | `100.00% <100.00%> (ø)` | | | [tests/qxmppomemodata/tst\_qxmppomemodata.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/483/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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[lnjX]

This broke the Qt 5.9 build. Could you fix it?

[melvo]

Thanks. #489 should fix it.