Add convenience data retrieval method to QXmppBitsOfBinaryDataList

[jbruechert]

PR check list:

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

[codecov]

Codecov Report

Base: 68.60% // Head: 68.24% // Decreases project coverage by -0.36% ⚠️

Coverage data is based on head (e62c02d) compared to base (f762f73).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
- Coverage   68.60%   68.24%   -0.37%     
==========================================
  Files         283      293      +10     
  Lines       24571    25108     +537     
==========================================
+ Hits        16857    17134     +277     
- Misses       7714     7974     +260     

Impacted Files	Coverage Δ
src/base/QXmppBitsOfBinaryData.cpp	90.90% <0.00%> (-9.10%)	⬇️
src/base/QXmppBitsOfBinaryDataList.h	100.00% <ø> (ø)
src/base/QXmppFileShare.cpp	45.33% <0.00%> (-16.94%)	⬇️
src/base/QXmppUtils.cpp	81.98% <0.00%> (-4.69%)	⬇️
src/base/QXmppFileMetadata.cpp	50.87% <0.00%> (-1.51%)	⬇️
src/client/QXmppFileEncryption.cpp	67.44% <0.00%> (-1.07%)	⬇️
src/base/QXmppMessage.cpp	98.31% <0.00%> (-0.55%)	⬇️
tests/qxmppmessage/tst_qxmppmessage.cpp	94.82% <0.00%> (-0.02%)	⬇️
src/base/QXmppError.h	0.00% <0.00%> (ø)
src/base/QXmppError.cpp	0.00% <0.00%> (ø)
... and 18 more

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[melvo]

@JBB There is a typo: It should be convenience :)