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

Changed VIEW::PrepareTargets() to more appropriate name VIEW::ClearTargets(). Added SELECTION layer to always-on-top list.

Mouse movement events are sent during autopanning, as the cursor position changes in the world coordinates (even if it stays still in the screen coordinates). It allows tools to update their state, as if the mouse was moved.

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.