make tiling drag configurable

fixes https://github.com/i3/i3/issues/5155

include/tiling_drag.h

@@ -9,6 +9,16 @@
  */
 #pragma once
 
+/**
+ * Tiling drag initiation modes.
+ */
+typedef enum {
+    TILING_DRAG_OFF = 0,
+    TILING_DRAG_MODIFIER = 1,
+    TILING_DRAG_TITLEBAR = 2,
+    TILING_DRAG_MODIFIER_OR_TITLEBAR = 3
+} tiling_drag_t;
+
 /**
  * Initiates a mouse drag operation on a tiled window.
  *