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)

visual_script_editor.cpp

@@ -2401,7 +2401,8 @@ RES VisualScriptEditor::get_edited_resource() const {
 }
 
 void VisualScriptEditor::set_edited_resource(const RES &p_res) {
-
+	ERR_FAIL_COND(script.is_valid());
+	ERR_FAIL_COND(p_res.is_null());
 	script = p_res;
 	signal_editor->script = script;
 	signal_editor->undo_redo = undo_redo;
@@ -2422,6 +2423,9 @@ void VisualScriptEditor::set_edited_resource(const RES &p_res) {
 	_update_members();
 }
 
+void VisualScriptEditor::enable_editor() {
+}
+
 Vector<String> VisualScriptEditor::get_functions() {
 
 	return Vector<String>();

visual_script_editor.h

@@ -295,6 +295,7 @@ public:
 	virtual void apply_code();
 	virtual RES get_edited_resource() const;
 	virtual void set_edited_resource(const RES &p_res);
+	virtual void enable_editor();
 	virtual Vector<String> get_functions();
 	virtual void reload_text();
 	virtual String get_name();