ENH: Add option to disable python wrapping on loadable modules

[RafaelPalomar]

This makes the python wrapping for loadable module components depend on the Slicer_USE_PYTHONQT CMake variable. This avoids python wrapping when Slicer_USE_PYTHONQT=OFF, which can save generation of files that won't be used.

NOTE: For discussion purposes, so far, this PR applies the concept only to the MRML components of the Colors module, but the idea would be to apply this across modules and components. Alternatively, the Slicer_USE_PYTHONQT variable could be checked on the CMake SlicerBuild macros.

@pieper, @jcfr would these changes make sense to you?

[jcfr]

Generally speaking, fixing this makes sense.

Instead of adding code in each module (and extension), I suggest to instead update the Slicer CMake macros to skip wrapping if the option Slicer_USE_PYTHONQT is set to OFF.

[RafaelPalomar]

Instead of adding code in each module (and extension), I suggest to instead update the Slicer CMake macros to skip wrapping if the option Slicer_USE_PYTHONQT is set to OFF.

That makes sense. I can work on it.