This MR rewrites the datepicker, fixing a number of issues around Kalendar while also making it behave like the other calendar views in the app. The month, year, and decade pickers are all infinitely scrollable views which are much better to use on mobile.
Additionally, the different weekview, monthview, etc. models have all been unified under a single InfiniteCalendarViewModel class, reducing duplicate code.
FWIW, this is the datepicker we should try to upstream to Kirigami Addons IMO.