[kcms] Port simple OverlaySheet headers to use title: property
requested to merge ngraham/plasma-desktop:port-simple-overlaysheet-headers-to-title-property into master
This was added to Kirigami in frameworks/kirigami!314 (merged) and simplifies and standardizes the creation of simple text-based headers in OverlaySheet. This commit ports plasma-desktop's KCMs to use them in all the places where it is possible to do so.
Tested; there are no visual changes.
cc @devinlin