Improve Kickoff's scrolling when changing categories

Use case: I'm in All Applications, I scroll down, select Utilities,
category switches inside Utilities and the scrollbar is at the same
bottom position from my first scroll, which means that I see the apps in
Utilities that are at the bottom instead starting from the top.

These are two changes: always scroll the view to top when going into a
subcategory and then always center on the parent category when going
level up. The breadcrumbs however store QModelIndex (which makes sense)
and QModelIndex cannot be used from QML, so I added a helper method to
the model that returns a row of the given index and the view then
centers on that row when going category up. Might not be good at all but
imho works&looks better.

REVIEW: 117757