[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 calender into the column to the right of the calendar, sharing space with the list of clocks

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

Calendar only:

Calendar + events:

Calendar + events (no events shown):

Calendar + time zones:

Calendar + events + time zones:

cc @teams/vdg