Optimize theme change in code editor

Postpone applying the whole theme when a setting changes, to avoid
updating everything many times when the whole editor theme is changed.

editor/code_editor.h

@@ -162,7 +162,10 @@ class CodeTextEditor : public VBoxContainer {
 	int error_line;
 	int error_column;
 
+	bool settings_changed = false;
+
 	void _on_settings_change();
+	void _apply_settings_change();
 
 	void _update_text_editor_theme();
 	void _update_font();