Added a Window node, and made it the scene root.

Still a lot of work to do.

modules/visual_script/visual_script_editor.cpp

@@ -38,7 +38,7 @@
 #include "editor/editor_node.h"
 #include "editor/editor_resource_preview.h"
 #include "editor/editor_scale.h"
-#include "scene/main/viewport.h"
+#include "scene/main/window.h"
 #include "visual_script_expression.h"
 #include "visual_script_flow_control.h"
 #include "visual_script_func_nodes.h"