QXmppHash: Add getters and setters

[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/xep.doc
• [ ] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #450 (0e34e16) into master (682636f) will decrease coverage by 0.02%. The diff coverage is 0.00%.

:exclamation: Current head 0e34e16 differs from pull request most recent head b1ce9bf. Consider uploading reports for the commit b1ce9bf to get more accurate results

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
- Coverage   69.04%   69.02%   -0.03%     
==========================================
  Files         263      263              
  Lines       23444    23454      +10     
==========================================
  Hits        16188    16188              
- Misses       7256     7266      +10     

Impacted Files	Coverage Δ
src/base/QXmppHash.cpp	0.00% <0.00%> (ø)
src/base/QXmppHash.h	0.00% <ø> (ø)

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