wmaker: Add window snapping feature.

This patch adds the ability to "snap" a window to one side of the screen by
dragging it to that side.  It is enabled by setting WindowSnapping = "YES" in
~/GNUstep/Defaults/WindowMaker.

Note that window snapping is automatically disabled if DontLinkWorkspaces =
"NO", as this feature also involves dragging a window to one side of the
screen.
WPrefs.app: Add ability to enable or disable window snapping.