wmaker: add directional window focus

This patch is adding directional window focus
(like in openbox), where the focus can be passed to
the current window neighboors with a shortcut key.
Comparisons are performed using window centres.

New options are FocusWindowLeftKey, FocusWindowRightKey,
FocusWindowUpKey, FocusWindowDownKey.
Other WM are setting those to SUPER+LEFT/RIGHT/UP/DOWN keys
but currently those are not set by default.

CirculateRaise option which is described in WPrefs as
'Raise window when switching focus with keyboard' is also
used to raise/not raise windows that are fully covered by
other windows.

src/window.h

@@ -405,6 +405,7 @@ WMagicNumber wWindowGetSavedState(Window win);
 
 void wWindowDeleteSavedState(WMagicNumber id);
 
+Bool wWindowIsFullyCovered(WWindow *wwin);
 Bool wWindowObscuresWindow(WWindow *wwin, WWindow *obscured);
 
 void wWindowSetOmnipresent(WWindow *wwin, Bool flag);