LibGUI: Add focus hooks to TextEditor

Author: https://github.com/alimpfard
Commit: c35493c156
Pull-request: https://github.com/SerenityOS/serenity/pull/2694
Issue: https://github.com/SerenityOS/serenity/issues/2685
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Libraries/LibGUI/TextEditor.cpp

@@ -1152,11 +1152,15 @@ void TextEditor::focusin_event(Core::Event&)
     m_cursor_state = true;
     update_cursor();
     start_timer(500);
+    if (on_focusin)
+        on_focusin();
 }
 
 void TextEditor::focusout_event(Core::Event&)
 {
     stop_timer();
+    if (on_focusout)
+        on_focusout();
 }
 
 void TextEditor::timer_event(Core::TimerEvent&)