KWatchGnuPG: Remove tray icon and quit app if window is closed

This fixes/avoids several problems caused by hiding the window on close:

• KWatchGnuPG's window wasn't shown when the tray icon was clicked.
• Selecting "GnuPG Log Viewer" in Kleopatra didn't have any effect, i.e.

KWatchGnuPG stayed hidden.

BUG: 384271