Multiple bug fixes

Changes since 0.8:

* Holding shift while using shape tool results in perfect square/circle/lines - try it!
* Paintbrush now remembers previous image size
* Saves as PNG by default, instead of BMP
* New option to choose from preselected sizes
* Undo/redo works with cut and paste
* Updated icons for shape fill styles
* Requested: Keyboard shortcuts for zoom in/out/actual
* Added: Bomb tool animation (also responds to Shift key)