MDANSE_GUI tray icon and send to background on window close.

**Is your feature request related to a problem? Please describe.**
In some cases we may want to leave the GUI running in the background during a MDANSE job.

**Describe the solution you'd like**
Similarly to other programs it might be good to allow the MDANSE_GUI to be sent to the background but leave it running. The MDANSE_GUI window will close but will be still running. The GUI will need to be closed from the tray icon. On window close it may be worth asking the user whether it want to send the process to the background or to close it entirely.

**Additional context**
See for example programs like Zoom which do not exit on window closing.