Requied for constrained Delaunay triangulation (P&S/ratsnest) and zone tesselation (GAL).

Redraws can be requested way too often than it is required. This commit adds redraw timeout:
- if the view became dirty and there has been no redraw for longer than certain time, it is redrawed immediately
- otherwise, we wait for the next frame

This in general improves smoothness of rendering.

Used in highlighting/routing code.

PCB items are refreshed on GAL switching (changes made using default renderer are displayed by GAL).

Added const to GetColor() and GetType() functions in WS_DRAW_ITEM.

Changed focus owner of LayerWidget to EDA_DRAW_PANEL_GAL to make keyboard events work (apparently everything works fine, to be tested more extensively).
Removed unnecessary event hook from EDA_DRAW_PANEL_GAL.

Smarter selection algorithm (does not allow to select both whole MODULE and its parts at the same time).
Cancel event works better (selection box does not appear after cancelling the selection tool).
Removed blinking selection box effect.
Model is accessed in more appropriate way (getModel() method).

More effective way of updating bounding boxes. IsCached() method made public. Removed some of unused fields from the layer description structure.