support moving window between heads using keyboard

Window Maker allows to perform practically all operations with windows
using only keyboard. One of the actions so far which required using
mouse was dragging window from one head (monitor) to another.

This patch introduces support for keyboard shortcuts. These shortcuts
move windows in circular fashion (if you have 3 and more monitors).

In case of 2 or 3 monitors arranged horizontally - window will just move
right/left.

In case of 3x3 setup - it is impossible to move window to central
monitor with keyboard.

- preserves window position and size (if display sizes are same)
- otherwise tries to fit window to smaller display

src/keybind.h

@@ -132,6 +132,10 @@ enum {
 	WKBD_WINDOW9,
 	WKBD_WINDOW10,
 
+	/* shortcuts to move window between heads */
+	WKBD_MOVE_12_TO_6_HEAD,
+	WKBD_MOVE_6_TO_12_HEAD,
+
 	/* launch a new instance of the active window */
 	WKBD_RELAUNCH,