Add possibility of redirecting the system tray "Back" button and use it to...

Add possibility of redirecting the system track "Back" button and use it to exit barcode page in clipboard applet

This removes the somewhat duplicate nested "back" button in the barcode page. When clicking the system tray's back button now, we first leave the barcode page and show the applet's main page; when clicking it again, we leave the applet again, as before. This is only active if the applet is shown in the system tray, if it is standalone, the back button is still shown.

Reason for the change

The duplicate back buttons look kind of bad, especially since they have the same icon. The navigation is also a bit confusing.

Screenshots or screen recordings

Before	After