Refactor the KNSQuick Page header and footer for Kirigami-ness

This removes the custom header implementation used on KNSQuick's Page component, which was originally implemented because Kirigami did not at the time support the granularity of control needed to do the stuff we wanted to be able to do. Now that this has changed, it was time to do a refactor and move the things into context actions that make sense to move there (which, essentially, is everything but the category selection, since that one is more type dependent, and arguably a part of the content of the page).

A dialog showing the Page, showing Wallpapers, with these changes:

The same dialog minus the changes:

The comic plasmoid feed view, which has no categories looks like this: