tiling drag: only start when there are drop targets (#5213)

This prevents potentially confusing drag & drop on fullscreen containers and
only-containers on workspaces.

fixes https://github.com/i3/i3/issues/5184

include/tiling_drag.h

@@ -9,6 +9,8 @@
  */
 #pragma once
 
+#include "all.h"
+
 /**
  * Tiling drag initiation modes.
  */
@@ -19,6 +21,13 @@ typedef enum {
     TILING_DRAG_MODIFIER_OR_TITLEBAR = 3
 } tiling_drag_t;
 
+/**
+ * Returns whether there currently are any drop targets.
+ * Used to only initiate a drag when there is something to drop onto.
+ *
+ */
+bool has_drop_targets(void);
+
 /**
  * Initiates a mouse drag operation on a tiled window.
  *