ui: Hide main window when closing with a backup running

Fixes #423 (closed)

The closing of the window would trigger a loop in which GtkWindow::close would call app_window::on_delete via signal handler, which in turn calls ui::quit, which calls GtkWindow::close

How did this ever work?

Hiding the window will work just fine, the window will be unhidden when opened again. And when the backup operation is done it will call ui::quit, which, when no backup is running, calls through to AdwApplication::quit