Add central position feature for active window

This patch adds a new Central feature under the window menu
"Other maximization" entry.
Shortcut can be configured via WPrefs "Center active window" action.
When called the active window is centered on the screen head.
If the window height or width are bigger than the head size,
the window is resized to fit.
There are some transitions defined as below:
*from fullscreen to center
*from any corner to center
*from top half to center top half
*from bottom half to center bottom half
*from left half to center left half
*from right half to center right half

Undoing the action is done via the window menu "Unmaximize" entry
or the shortcut.

src/menu.h

@@ -40,6 +40,7 @@
 #define MI_SNAP_BL	14
 #define MI_SNAP_BR	15
 #define MI_SNAP_TILED	16
+#define MI_CENTRAL	17
 
 typedef struct WMenuEntry {
 	int order;