Enable Drag and Drop for SubViewports and Windows

Search embedded Windows and SubViewports iteratively for Drop targets.
Display Preview above embedded windows.
Store Drag-information windowmanager-window-wide or application-wide.
Send mouse enter/exit notifications to Drop-targets.

Author: Sauermann

scene/gui/control.cpp

@@ -1850,7 +1850,7 @@ void Control::force_drag(const Variant &p_data, Control *p_control) {
 void Control::set_drag_preview(Control *p_control) {
 	ERR_FAIL_COND(!is_inside_tree());
 	ERR_FAIL_COND(!get_viewport()->gui_is_dragging());
-	get_viewport()->_gui_set_drag_preview(this, p_control);
+	get_viewport()->_gui_set_drag_preview(p_control);
 }
 
 bool Control::is_drag_successful() const {