Various minor UI improvements

- Clean up layer toolbox buttons and add tooltips.
- When deleting a layer, activate the layer that appears at the same
position on-screen (rather than activating the layer with the same index
number).
- Much faster viewport refreshes when just changing the active layer.
(We don't need to recomposite the image, so skip that step entirely.)
- Faster restoration of the default cursor after an action has
completed.  Amazing how something as small as this makes the program
feel snappier.