Find dependencies dependencies of QXmpp when finding QXmpp

[jbruechert]

This way, users of the library don't need to find QtNetwork and QtXml themselves before finding QXmpp

PR check list:

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

[codecov[bot]]

Codecov Report

Merging #434 (22476cc) into master (be310d6) will decrease coverage by 3.49%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   79.57%   76.08%   -3.50%     
==========================================
  Files         237      235       -2     
  Lines       23881    20724    -3157     
==========================================
- Hits        19004    15768    -3236     
- Misses       4877     4956      +79     

Impacted Files	Coverage Δ
src/client/QXmppClientExtension.cpp	71.42% <0.00%> (-28.58%)	:arrow_down:
tests/qxmppmixiq/tst_qxmppmixiq.cpp	73.72% <0.00%> (-26.28%)	:arrow_down:
tests/qxmppiq/tst_qxmppiq.cpp	77.77% <0.00%> (-22.23%)	:arrow_down:
tests/qxmpprosteriq/tst_qxmpprosteriq.cpp	79.77% <0.00%> (-20.23%)	:arrow_down:
tests/qxmppresultset/tst_qxmppresultset.cpp	80.39% <0.00%> (-19.61%)	:arrow_down:
tests/qxmpphttpuploadiq/tst_qxmpphttpuploadiq.cpp	84.31% <0.00%> (-15.69%)	:arrow_down:
tests/qxmppmammanager/tst_qxmppmammanager.cpp	84.61% <0.00%> (-15.39%)	:arrow_down:
...ereceiptmanager/tst_qxmppmessagereceiptmanager.cpp	85.00% <0.00%> (-15.00%)	:arrow_down:
tests/qxmpparchiveiq/tst_qxmpparchiveiq.cpp	86.66% <0.00%> (-13.34%)	:arrow_down:
tests/qxmppvcardiq/tst_qxmppvcardiq.cpp	88.76% <0.00%> (-11.24%)	:arrow_down:
... and 166 more

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