kconfig_compiler: Allow generating QML type registration macros
This adds a new option (QmlRegistration) that generates the needed code for declarative QML type registration
When combined with the existing singleton option a QML singleton is registered
See network/alligator!94 for a usage.
Limitations:
Currently when using there will be build warnings like
Warning: alligatorsettings.h:: KConfigSkeleton is used but cannot be found.
This is because the metatypes file for KConfig is not available. See https://bugreports.qt.io/browse/QTBUG-123052
The result will still be useful since it allows qmllint/qmlls to know about the config's properties etc. However some type information will be missing and qmlcachegen will not be able to generated C++ code for it.
That's not really a blocker for merging this though