Add extra class for HttpFileSource instead of using QUrl

[lnjX]

Before just QUrl was used, which was okay. This should make it better recognizable and it makes clear it is only used for HTTP urls.

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

Merging #461 (28a9f41) into master (f672197) will decrease coverage by 0.00%. The diff coverage is 63.15%.

:exclamation: Current head 28a9f41 differs from pull request most recent head 9b5d045. Consider uploading reports for the commit 9b5d045 to get more accurate results

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   68.64%   68.63%   -0.01%     
==========================================
  Files         273      274       +1     
  Lines       23891    23903      +12     
==========================================
+ Hits        16400    16407       +7     
- Misses       7491     7496       +5     

Impacted Files	Coverage Δ
src/base/QXmppFileShare.h	100.00% <ø> (ø)
src/base/QXmppHttpFileSource.cpp	61.53% <61.53%> (ø)
src/base/QXmppFileShare.cpp	62.26% <66.66%> (-1.38%)	:arrow_down:
tests/qxmppomemodata/tst_qxmppomemodata.cpp	98.25% <0.00%> (+<0.01%)	:arrow_up:

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