Hot corners feature core

Add mouse pointer position detection to trigger the corner actions.

Screen corners can be assigned an external command to be
executed when the mouse pointer is entering those areas.

In WPrefs, "Hot Corner Shortcut Preferences" can be used
for configuration or by manually adding a "HotCorners" key
and value to "YES" in the ~/GNUstep/Defaults/WindowMaker file.

Actions are specified by the "HotCornerActions" and are defined
as a four entries list ("top left action", "top right action",
"bottom left action", "bottom right action").
A screen corner area is a cube shape defined by the "HotCornerEdge"
which is a number of pixels from 2 (by default) to 10.

To lower the risk of triggering that feature accidentally a
"HotCornerDelay" key can be used which is the time before the action
is triggered while the pointer is in one of the screen corner.
Default value is 250 ms.

Hot Corners feature is disabled by default.

src/WindowMaker.h

@@ -474,6 +474,11 @@ extern struct WPreferences {
 
 	char show_clip_title;
 
+	char hot_corners;                  /* let corners execute actions */
+	int hot_corner_delay;		   /* Delay after which the hot corner is triggered */
+	int hot_corner_edge;		   /* Hot corner edge size */
+	char *hot_corner_actions[4];	   /* Action of each corner */
+
 	struct {
 #ifdef USE_ICCCM_WMREPLACE
 		unsigned int replace:1;               /* replace existing window manager */