Allow resizing from top and left; Enlarge grip in bottom corner; ...

Resubmitting my pull request against the atlas branch.

### Summary of Changes
##### 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 bugs
When resizing very quickly, sometimes the mouse moves too far before the app finishes resizing. Adding fallthrough else-clauses to the drop handlers fixes this.