[facets] Fix disappearance of navtop component on facet filtering
which is because facets are replacing the whole #pageContent div, while this one
contains other stuff than the view:
* a type selector component that should be dropped for a while,
* a computed title,
* the page navigation.
Then the view content itself is in a #contentmain div. The thing is that the
navigation should be rebuilded on filtering (this is not the case for other
bullets in the list above). This is currently handled specifically in the
ajaxcontroller (except for the type selector which will disappear... who said it
should be dropped at once?).
So to fix this we:
* put the page navigation into the "contentmain" div
* don't replace anymore "pageContent" but "contentmain"
After that we can even remove from the ajax controller the code that
reimplements title handling similarly to the main template.
Notice the part that changes the main template has to be ported to squareui.
Closes #17074195