[kcm/translations] Port OverlaySheet header to use title: property
requested to merge ngraham/plasma-workspace: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 the Translations KCM's OverlaySheet header to use it. This is the only place in plasma-workspace where such a port is appropriate.
Tested; there are no visual changes.
cc @devinlin