Remove QSignalMapper and replace by direct lambda calls

[codeling]

QSignalMapper has undergone changes from Qt 5 to Qt 6; and actually it is not required anymore, because lambdas can be used (since Qt5) instead to directly do the mapping.

Therefore, I'm proposing to consistently use lambdas instead of QSignalMappers; this would simplify code because:

• It would reduce lines of code because we only need a single signal/slot connection instead of :
  • link a signal to the QSignalMapper
  • the actual slot to the QSignalMapper's mapped(String/Int/...) signals connection
  • the insertion of the mapping
• It would make it unnecessary to distinguish between Qt5 and Qt6 via #if 's

[codeling]

The two usages in current code are:

• Libs/Widgets/ctkSettingsPanel.cpp, it can't easily be removed here, because that would require an API change (using function pointer syntax for registerProperty) - in an intermediate step, a version with method pointer parameter should probably be introduced and the other version with char const * signal be deprecated.
• ctkSignalMapper - this class should probably be deprecated in favor of users of the class directly using lambdas