Redesign calendar
This MR does three things:
- Make the header component optional (this allows custom implementation like in https://invent.kde.org/plasma/plasma-workspace/-/merge_requests)
- Use buttons to navigate to year/month/decade view instead of a non-discoverable click on the heading
- Switch from a StackView to a SwipeView providing better touch navigation
- Remove the lines in the calendar view
Edited by Carl Schwan