Added generation of CancelTool event upon ESC key press.
Minor bugfix.

Changed reaction of SELECTION_TOOL to ToolCancel event (first event clears selection, second one deactivates the tool).

Smarter way of the overlay rendering (overlay is always refreshed, while cached&noncached targets only if the viewport or items have changed).

Moved VIEW_ITEM::ViewGetRequiredLayers() functionality to the VIEW class. Now required layers are set per layer basis instead of per item.