[OverlaySheet] Add title property
The vast majority of overlaysheet headers are simply headings, so this provides an easy way to specify sheet titles in a consistent way across applications (supporting eliding and tooltips when truncated as well). Setting the header property will override the default heading, and so title would have no effect in this case.
I also used level 2 headings here.
Example in kirigami gallery (switching to title property):
Edited by Devin Lin