GAL is strict regarding which layers are rendered, therefore to show
ratsnest lines when an item is dragged - the ratsnest layer must be kept
enabled. Ratsnest line visibility is enabled per item and this is the
way to show/hide the lines.
Fixes: lp:1666473
* https://bugs.launchpad.net/kicad/+bug/1666473
Colors are assigned to virtual layers while the ToLayer_ID returns a
physical layer enum. The GetLayerColor/SetLayerColor take LAYER_NUM
(int) values that can be larger than the physical layer count.
Also adds indicators in layers palette for feedback.
Also generates sized images for all indicators instead of using
scaled bitmaps (which didn't look great).
Also fixes a completely unrelated typo in a UI string.