Selection tool

[foxnne]

One critical thing is adding a selection tool so it's easy to copy/paste parts of an image around. This is decently complex, as we will need to track changes for history as well as making sure we don't break other tools.

Currently, all that is available is copying the selected sprite to another tile (ctrl/cmd + arrow keys) or shifting the currently selected sprite (shift + arrow keys). Shifting isn't perfect as it leaves existing pixels as they are when shifting.

I think I like these quick keys for copying sprites, which comes in handy when creating and editing animations, but a proper selection tool would be really nice.