Add support for minimizing to system tray on startup

If the user wants to automatically launch NeoChat when the system
starts up, the user may also want to minimize the window to system tray
on startup. So a new option named "Minimize to system tray on startup"
is added.

The option is only visible on desktop platforms, and is only enabled
when "Close to system tray" is checked.

In order to restore window geometry for the first time the user opens
the window if the option is checked,

1. a new function named `restoreWindowGeometry` is added, and
   `restoreWindowGeometryConnections` will be enabled if the option is
   checked, and will be disabled after the window debuts.
2. `saveWindowGeometryConnections` will be enabled if the option is not
   checked, and will be disabled if checked and enabled after the window
   debuts.