Make it possible to compile gtk2 gtk3 qt kde versions all at the same time in order to be more friendly to packagers

Add three new options to enable the four backends at the same time (I agree ENABLE_KDE_QT is a funny name, plz feel free to change it .... :tongue:)

In order not to break old build scripts, I only changes the target names (to avoid name conflicts between different backends) when the new options (disabled by default) are enabled.

Tested with:
1. gtk2 only (without new options)
2. gtk3 only (without new options)
3. gtk2 + gtk3
4. qt only (without kde support) (without new options)
5. kde only (qt with kde support) (without new options)
6. qt + kde (qt with and without kde support)

All build fine and the file names (as well as executable/package names in .desktop file/python wrapper script) looks correct (haven't really install and run though.).