Fix resizing

##### Allow resizing from top and left
Only the two edges were added. No additional corner resize grips were added.

##### Enlarge grip in bottom corner
This was really hard to use with a stylus on my Surface, so I increased it from 11x11 pixels to 22x22.

##### Fix subtle resizing bug
When resizing very quickly, sometimes the mouse moves too far before the app finishes resizing. Adding fallthrough else-clauses to ResizeDrop_DragDelta, ResizeRight_DragDelta, and ResizeBottom_DragDelta fixes this.