- themes: move user-themes to /var/tuxbox/themes ...

... because the other user-stuff (icons, locale, plugins) is also
stored in /var/tuxbox.
The directory /var/share/tuxbox/neutrino/themes will be removed.
The parent directories should now be obsolete and can be removed too.
Merge remote-tracking branch 'cool-cst-next/cst-next'

Conflicts:
	src/gui/themes.cpp