Use dock to determine whether drawer is on right side when swapping

Previously, we assumed that it always switched from left to right or
vice versa when calling swapDrawer.  However, now we may also call
swapDrawer when changing the value of "KeepDockOnPrimaryHead", which
wouldn't actually switch which side of the screen it's on.

So instead, we determine which side of the screen it should be on
based on the dock.

src/dock.c

@@ -4610,7 +4610,7 @@ static void swapDrawer(WDock *drawer, int new_x)
 {
 	int i;
 
-	drawer->on_right_side = !drawer->on_right_side;
+	drawer->on_right_side = drawer->screen_ptr->dock->on_right_side;
 	drawer->x_pos = new_x;
 
 	for (i = 0; i < drawer->max_icons; i++) {