[Qt] Done the QNotepad Application

Try to build a simple notepad application based on Qt.

I use the QTextEdit widget to handle the inputted by user.

Because a simple window may have a series of menu, I implement the QAction and QMenu to handle the actions from menu.

In this work, I can learn how to build menus and to handle these actions.

I also learn how to connect between signal and slot from this work.

Here is the demo video,