qml: Respect user theme settings

[quarnster]

Not to be confused with syntax color schemes. I'm talking about the styling of the quick panel, view tabs, side bar etc.

See http://qt-project.org/wiki/QmlStyling, #119 (comment), and the various actual sublime text themes.

I think the theme properties available were never documented properly so expect having to trial'n'error a bit. Some bits from old code might be useful.

[erbridge]

Migrated to limetext/lime-qml#10.