Add a sidebar with a date chooser and an agenda view

This starts implementing the new adaptive design from https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/calendar/adaptive-calendar.png.

This:

• 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.

CC @snwh