Allow snapping a window to screen corners

[tokyo4j]

My idea is adding a setting like <snapping cornerRange="10%"> and allow snapping a window to corners if the cursor is in the red region of outputs in the following figure.

[Bezzumnbiu]

Good idea, I wrote about adding this thing earlier

[johanmalm]

I like the sound of this.
Best to merge this after 0.7.2

[tsujan]

I used it a lot with kwin_x11. When I saw that it was missing in labwc, I defined right click actions on title-bar buttons for snapping to corners, but a direct mouse snapping will be much more comfortable.