[applets/digital clock] Make pop-up layout more compact and consistent

We got some user complaints about the new Calendar pop-up, saying that
it used too much space, which honestly is a reasonable critique.

This MR tweaks the layout in the following ways:
- Reduce paddings from units.largeSpacing to units.smallSpacing
- Replace the huge custom date header with a standardized PlasmoidHeader
  containing the current locale's standard long date and the pin button
- Move the events view from under the calendar into the column to the
  right of the calendar, sharing space with the list of clocks
- Increase space efficiency of Events view by removing the per-event-type section headers

Hopefully this should address concerns with the 5.19 layout. I think it
does look much nicer, myself.