Use unique_ptrs

[lnjX]

• Replace raw pointers by unique_ptr
• RegistrationManager: Replace QScopedPointer with unique_ptr

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

[codecov[bot]]

Codecov Report

Merging #443 (83e5467) into master (6bd74d6) will decrease coverage by 0.00%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #443      +/-   ##
==========================================
- Coverage   69.29%   69.28%   -0.01%     
==========================================
  Files         254      261       +7     
  Lines       23377    23362      -15     
==========================================
- Hits        16198    16187      -11     
+ Misses       7179     7175       -4     

Impacted Files	Coverage Δ
src/client/QXmppAtmTrustMemoryStorage.h	100.00% <ø> (ø)
src/client/QXmppAttentionManager.cpp	80.64% <ø> (-0.61%)	:arrow_down:
src/client/QXmppAttentionManager.h	100.00% <ø> (ø)
src/client/QXmppBookmarkManager.cpp	0.00% <ø> (ø)
src/client/QXmppBookmarkManager.h	0.00% <ø> (ø)
src/client/QXmppClient.cpp	59.80% <ø> (-0.27%)	:arrow_down:
src/client/QXmppClient.h	93.75% <ø> (+0.41%)	:arrow_up:
src/client/QXmppClientExtension.cpp	78.94% <ø> (-2.01%)	:arrow_down:
src/client/QXmppClientExtension.h	100.00% <ø> (ø)
src/client/QXmppDiscoveryManager.cpp	56.03% <ø> (-0.75%)	:arrow_down:
... and 29 more

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