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.