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.

pcbnew: hooked Tool Framework into the edit panel. Added a sample selection tool (not fully functional).

Added core tool framework files + boost::context assembly stuff (initial, very buggy and unfinished work).