This starts implementing the new adaptive design from https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/calendar/adaptive-calendar.png.
- adds a sidebar with a date chooser and an agenda view
- removes the back and forward buttons
- removes the year view
- doesn't make the window adaptive
- doesn't implement the timeline view (which is there mostly for the mobiles)
Ensuring the app is properly adaptive and works well on phones is going to be a beast of its own, so I left that aside for another time: ensuring the sidebar works as intended is complex enough.