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

VIEW_ITEMs save the layer numbers they use, it allowed to speed up removal of items.

Added flipping mode.

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).