pdInput: add ESC to Edit key bundle

Escape is not technically an edit key, but it fits in with the likes of
Enter, Space Bar, etc, so I'm sticking it there.
pdCanvas: add basic key modifiers for selections

- ESC to remove current selection
- DEL to erase currently selected area

Also cleaned up some old code tied to the old Undo engine, which I
hadn't noticed until now.
Layer toolbox: add support for Edit key bundle

Note that these shortcut keys are also supported on the main canvas
while the Move/Resize tool is in use!

DEL: delete current layer
INS: raise Add New Layer dialog
TAB / Shift+TAB: advance through the layer stack
SPACE BAR: toggle layer visibility