[GSOC]Use Widget Layout for the Start Center Weekly Update 03

Hi all!

Since my last update I have:

- Finally managed to convert the Start Center to use .ui, with help from
Caolan (thanks again). There are still some details to be worked out
though, like how the three smaller icons on the bottom should remain as a
toolbar, or be just regular buttons. Comments in the code seem to indicate
that they were put in a toolbar as workaround for rendering them correctly.
Adding them as regular PushButtons seem the right way, since that's more
unified. (https://gerrit.libreoffice.org/#/c/4502/)
- Started looking at the problem of adding thumbnails for recent docs. I
found thumbnail loading code that has been copied and pasted 2 times
already, so instead of copying it for a 3rd time I refactored a bit. (
https://gerrit.libreoffice.org/#/c/4779/)

This week I'm planning to start adding thumbnails for the recently used
docs.

All the best,
Krisztian