Add a ListBox HeaderHeight property so headers can be shorter than rows

A ListBox header row currently has no height of its own: on web the header is always exactly as tall as a row, and on desktop it auto-sizes to the same height as the rows. That wastes vertical space when the header exists only to provide the column-resize dividers — for example, leaving HasHeaders = True with blank header text still consumes a full row's height.

Add a HeaderHeight property to the desktop and Web ListBox. HeaderHeight = 0 (the default) keeps today's behaviour where the header matches the row height, while a positive value pins the header to that height. A ListBox with tall rows can then have a slim header strip that is still big enough to grab the column dividers and resize columns.