Added option for ability to move half-maximized between heads.

Added new option to Window Maker preferences to enable half-maximized
windows movement on all available heads. Option can be found in WPrefs
app on Expert section.

WPrefs.app/Expert.c

@@ -100,6 +100,9 @@ static const struct {
 	{ N_("Snapping a window to the top maximizes it to the full screen."),
 	  /* default: */ False, OPTION_WMAKER, "SnapToTopMaximizesFullscreen" },
 
+	{ N_("Allow move half-maximized windows betweem multiple screens."),
+	  /* default: */ False, OPTION_WMAKER, "MoveHalfMaximizedWindowsBetweenScreens" },
+
 	{ N_("Open dialogs in the same workspace as their owners."),
 	  /* default: */ False, OPTION_WMAKER, "OpenTransientOnOwnerWorkspace" }