gtkui: make appearence (fonts/colors) more configurable

This is quite a huge patch for the gtkui plugin and I'm not sure if I did everything as you'd expected, so I am happy about any feedback/critisism. 

Short summary about what this patch is supposed to do:

- allow font customization for listview, listview header and tabstrip widget
- restyle and rename listview and tabstrip color preferences
- add further color customizations (tabstrip: selected, playing, listview: group title)
- remove embolden current track checkbox from current location since this can now be configured in Preferences -> Appearance -> Playlist