Make the calendar a native widget instead of a custom widget (#2457)

Adding custom calendar widget as native widget type without the custom
widget infrastructure.

Biggest change is how events are drawn. Old widget listened on
`onRecords` and rebuilt all events on every change, so single cell edit
costs as much as whole table. Now rows travel a pipeline (CalendarSource
-> FilteredRowSource -> CalendarRenderer), so only rows in the visible
range (calendar perspective) are drawn and updated.

Second big change is in Day and Week, which are also capped to 20 events
per day with "+N more" label, because Toast UI draws all of them and
doesn't have the ability to render "+ N more" as it do in the month
view.

TUI form and detail popups are not used anymore, double click opens
Grist Record Card and drag on empty grid creates a row and opens the
card.

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Co-authored-by: Paul Fitzpatrick <paulfitz@alum.mit.edu>