Portals - lift roomlist restrictions and fix link bug

Allows users to have the RoomManager as the roomlist.

Fixes a couple of bugs dealing with situations where users attempt to link Portals to Rooms outside the roomlist.

Adds a PortalEditorPlugin allowing you to flip individual portals.

editor/plugins/room_manager_editor_plugin.h

@@ -33,6 +33,7 @@
 
 #include "editor/editor_node.h"
 #include "editor/editor_plugin.h"
+#include "scene/3d/portal.h"
 #include "scene/3d/room.h"
 #include "scene/3d/room_manager.h"
 #include "scene/resources/material.h"
@@ -89,4 +90,29 @@ public:
 	~RoomEditorPlugin();
 };
 
+///////////////////////
+
+class PortalEditorPlugin : public EditorPlugin {
+	GDCLASS(PortalEditorPlugin, EditorPlugin);
+
+	Portal *_portal;
+	ToolButton *button_flip;
+	EditorNode *editor;
+
+	void _flip_portal();
+
+protected:
+	static void _bind_methods();
+
+public:
+	virtual String get_name() const { return "Portal"; }
+	bool has_main_screen() const { return false; }
+	virtual void edit(Object *p_object);
+	virtual bool handles(Object *p_object) const;
+	virtual void make_visible(bool p_visible);
+
+	PortalEditorPlugin(EditorNode *p_node);
+	~PortalEditorPlugin();
+};
+
 #endif