[GSOC]Use Widget Layout for the Start Center Weekly Update 11-12

Hi all!

Since my last update I made it so deleted, local files automatically get
removed from recent docs, and toggles for templates only show up if there
are templates for that filetype.
I also did some tweaks to Start Center based on some suggestions in UX, to
see how it would look: https://gerrit.libreoffice.org/5901/

I changed ThumbnailView to make scrolling smoother, and not per-line. I
tried to do this in a way that won't horribly break everything else in
ThumbnailView. https://gerrit.libreoffice.org/5951

All the best,
Krisztian