qt: add setting to hide the window on close
When qt-hide-window-on-close
is set,
the application would not exit on close
event, but rather the window becomes
hidden.
This setting is not enabled by default,
and is only applicable when qt-system-tray
is also set.
I believe that with the media library, this setting becomes relevant to keep one instance open rather than closing and re-opening. Currently, even if the user does not want to close the application, the inclination is to close the application since the window occupies space in the task bar.