Add initial support for per-urgency frame-colors

This is my first implementation for per-urgency frame-colors.

I tried to change as few things as possible, but it still needed some tweaking of x.c to get the overlapping of notifications with higher urgency over the ones with lower urgency working.
Also the new frame-colors are totally optional, without them given in the [urgency_*] sections it automatically falls back on the one color in the [frame] section.

closes #154