Make rect resize corner dependent on mouse position in rect

[herr-biber]

Currently, using the right mouse button, rects will be resized from the lower right corner.

New functionality:

• if mouse in upper half, change rect to the top. Keep bottom border position.
• if mouse in lower half, change rect to the bottom. Keep bottom border position.
• ... same for left and right.

The current configuration behaves like the lower right corner case.

[herr-biber]

commit c2663dc841f1ee0ef9786a02edcdd377ec8ff78a Author: Markus Roth markus.roth@herr-biber.de Date: Sun Mar 11 01:58:16 2012 -0800

Fix Issue #70: Make rect resize corner dependent on mouse position in rect.