[overlaysheet] Use more conditional positioning for close button
requested to merge ngraham/kirigami:improve-overlaysheet-header-close-button-positioning into master
If the header is very tall, we should put the header in the top-right corner. But we want to ensure that it remains vertically centered for short or single-line headers.
Here's how it looks now with various header heights:
BUG: 430581 FIXED-IN: 5.78
cc @teams/vdg