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/code_editor.cpp

@@ -1456,8 +1456,6 @@ void CodeTextEditor::set_edit_state(const Variant &p_state) {
 			text_editor->set_line_as_bookmark(bookmarks[i], true);
 		}
 	}
-
-	text_editor->grab_focus();
 }
 
 void CodeTextEditor::set_error(const String &p_error) {