[GSOC]Use Widget Layout for the Start Center Weekly Update 05

Hi all!

Since my last update I was working on the custom widget for thumbnails.
After updating my master, and recompiling, it came to light that I
introduced circular dependencies, which I've been trying to resolve ever
since.
I wanted to use the class ThumbnailView (
http://opengrok.libreoffice.org/xref/core/include/sfx2/thumbnailview.hxx)
in BackingWindow. ThumbnailView is in sfx2 and BackingWindow is in
framework. Sfx2 depends on framework, so I can't just simply include it.
Kendy told me to move the class. I tried putting it in vcl, since UI stuff
is in there, and both framework and sfx2 uses it, but ThumbnailView uses
stuff from drawinglayer, which depends on vcl, so that didn't work.
I thought about putting it in framework, but I'm not sure it'd logically
belong there. Also, I found the structure of framework somewhat confusing,
and it seems to me that the only submodule that exports things globally is
fwe, where ThumbnailView doesn't seem to belong either (and sorry if I'm
using any terms horribly wrong here).

I'd be really glad if someone would be kind enough to give some suggestions
about resolving this, because I'm stuck.

All the best,
Krisztian