need to port: GNOME Calendar

As mentioned in the dev docs (https://developer.puri.sm/Design/Apps_overview.html), GNOME Calendar will be the calendar offering in the Librem 5.

Anyone wishing to work on porting this to the Librem 5 should sign up for this task and reach out with questions in the community matrix rooms.

This issue needs some initial investigation to identify actually what needs to be done to port this application and make it look good on the phone screen resolution.