Add GUI application

[changkun]

There are two options:

• Cross-platform GUI app
• Tray in system menu

Let's see which we will decide.

[changkun]

It looks like Gio is still under major rethinking on its API design https://todo.sr.ht/~eliasnaur/gio/555.

Maybe Fyne could be a candidate because it supports systray (https://github.com/fyne-io/systray) well on all platforms.