Clean-up and preparation for JamomaConfig.cmake
Make a package for Jamoma, and export targets

This allows target_link_libraries call to import the required includes.
For instance doing target_link_libraries(DSP) would also include_directories the DSP includes.

Also, external software can now do target_link_libraries(Jamoma::DSP) like Qt does (to be tested).
This will also import the includes.
However this breaks since the targets change from JamomaX to X.

TODO : make a function in JamomaConfig.cmake to import the extensions.