[applets/kickoff] Make section headers more section headery
requested to merge ngraham/plasma-desktop:make-kickoff-section-headers-more-section-headery into master
The header text uses a PlasmaExtras.Header component, but with the same size as the text of the list items. We typically use larger text for section headers to distinguish them. So let's do that.
This is an intentionally minimal change to be backportable and avoid an argument stemming from anyone not liking the way section headers look and proposing a new design. :)
BUG: 434669 FIXED-IN: 5.21.4
cc @teams/vdg