[Feature Request] Open main window when double clicking tray icon/or opening defguard-client even if its already running

[Brasse83]

Many of my users get confused if they have the client running or not.
If the main window would show when double clicking tray icon and/or if the main window would show when running defguard-client.exe again even if its already running, it would be much clearer.

[teon]

Depends on: #509