Handful of minor & (hopefully) uncontroversial improvements to calendar

## Improvement One

Better zoom animation w/ our usual easing treatment. Same animation as plasma-workspace!400 (merged), but with a less steep zoom (items go to 2x size before fading out instead of 3x size)

Improvement Two

More consistent font size. Just using a plain PlasmoidHeading w/out any extra spice results in an acceptable appearance that's consistent both within itself (no more wonky month sizing) and with other plasmoids.

## Improvement Three

The "go to higher view" button now has a go-up icon and has become a toolbutton, which makes it look clickable.

(Collision w/ grid not added by this MR; existing bug)