[Sidebar view] Port headers to use Kirigami.AbstractApplicationHeader

This is the correct thing to do in general because custom views that replicate standard components are bad. It also ensures that the headers will change their colors properly as the color scheme changes once we merge the Tools Area work.

The only Visual/UX change is that the subcategory header is made more standard in appearance and behavior, with a discrete back button instead of the entire area being clickable. This represents a mild regression in usability since the clickable area is smaller now, but it's a win for consistency and code simplicity.

cc @teams/vdg