Allow tiling views to be dragged by the titlebar

Enables titling views to be dragged by the titlebar. This is in addition
to using the modifier and dragging them from anywhere on the container
surface. Floating views already allow this behavior.

sway/input/cursor.c

@@ -942,8 +942,9 @@ void dispatch_cursor_button(struct sway_cursor *cursor,
 	}
 
 	// Handle moving a tiling container
-	if (config->tiling_drag && mod_pressed && state == WLR_BUTTON_PRESSED &&
-			!is_floating_or_child && cont && !cont->is_fullscreen) {
+	if (config->tiling_drag && (mod_pressed || on_titlebar) &&
+			state == WLR_BUTTON_PRESSED && !is_floating_or_child &&
+			cont && !cont->is_fullscreen) {
 		seat_pointer_notify_button(seat, time_msec, button, state);
 		seat_begin_move_tiling(seat, cont, button);
 		return;