Optimized ScriptEditor initialization when many scripts are loaded

This change avoids the editor to freeze for several seconds when a
project with lots of scripts is loaded in the editor.

It focuses on a few heavy operations previously executed on all
previously loaded scripts:
- Initialize script resource (script validation/parsing) only
on focus
- ScriptTextEditor: code editor and edit menu are added to the
scene only on focus
- Add to recent scripts only when opening new scripts
(load/save scene metadata)

editor/plugins/script_editor_plugin.h

@@ -90,6 +90,7 @@ public:
 	virtual RES get_edited_resource() const = 0;
 	virtual Vector<String> get_functions() = 0;
 	virtual void set_edited_resource(const RES &p_res) = 0;
+	virtual void enable_editor() = 0;
 	virtual void reload_text() = 0;
 	virtual String get_name() = 0;
 	virtual Ref<Texture> get_icon() = 0;