Add ability to run the app in the background

This pull request is mostly an illustration of what I was asking for yesterday (#153 (closed)). I apologize as I wasn't really clear yesterday: there's no need to add a system tray implementation or something like that as we already have everything to be able to play music in background (as you can see in the code) in the app's current state.

To describe a little bit more what the code here is actually doing, if a song is in the queue, when the app is closed, the main window is hidden. To show the window again, we just click on the notification shown by GStreamer or in the "Background activity" menu, which can show the app if it is installed as a Flatpak. To completely quit the app, even if there's a song in the queue, we simply use the existing keyboard shortcut Ctrl+W or Ctrl+Q. If there's no song in the queue, the app will be completely closed.

This matches the default behavior of most existing music players, especially music players from the GNOME ecosystem.