application: Several improvement

application: Remove redundant code

Due to "win.show-help-overlay" shortcut already provided by GTK, we don't need to define it again.

data: Remove redundant accel entries

GTK4 can show accels without accel entries.

help-overlay: Convert accellerator to action-name

With action-name, we don't need to define accelerators again.

application: Add tooltip to Main Menu

To comply GNOME HIG add tooltip to main menu. More information: https://developer.gnome.org/hig/patterns/feedback/tooltips.html

application: Implement quit action

Implement Quit action to comply GNOME HIG. More information: https://developer.gnome.org/hig/reference/keyboard.html