Allow adding multiple oob urls and a description

[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: 68.61% // Head: 68.62% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (2889c12) compared to base (6156452). Patch coverage: 87.50% of modified lines in pull request are covered.

:exclamation: Current head 2889c12 differs from pull request most recent head cc5b61e. Consider uploading reports for the commit cc5b61e to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #476 +/- ## ========================================== + Coverage 68.61% 68.62% +0.01% ========================================== Files 283 285 +2 Lines 24573 24603 +30 ========================================== + Hits 16860 16885 +25 - Misses 7713 7718 +5 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/476?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/476/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/QXmppMessage.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/476/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBNZXNzYWdlLmNwcA==) | `98.31% <80.00%> (-0.55%)` | :arrow_down: | | [src/base/QXmppOutOfBandUrl.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/476/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBPdXRPZkJhbmRVcmwuY3Bw) | `91.30% <91.30%> (ø)` | | | [src/base/QXmppOutOfBandUrl.h](https://codecov.io/gh/qxmpp-project/qxmpp/pull/476/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL2Jhc2UvUVhtcHBPdXRPZkJhbmRVcmwuaA==) | `100.00% <100.00%> (ø)` | | | [tests/qxmppmessage/tst\_qxmppmessage.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/476/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBtZXNzYWdlL3RzdF9xeG1wcG1lc3NhZ2UuY3Bw) | `94.82% <100.00%> (-0.02%)` | :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)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.