[GSoC] Use Widget Layout for the Start Center

Hi all!

I was happy to see that my GSoC proposal has been accepted, thanks for the
opportunity.

I have done some work on LO before, implementing a small autosave feature,
doing some code cleanup, and converting dialogs to glade format.

My proposal is this one:
https://wiki.documentfoundation.org/Development/Gsoc/Ideas#Use_Widget_Layout_for_the_Start_Center

"Currently, the Start Center (the start screen you get when you start
LibreOffice) is only a static bitmap, with few buttons and links. It would
be useful to present few recently used documents there (as thumbnails), and
do more fancy stuff. (...) [C]ode for rendering the thumbnails will have to
be introduced, and maybe some code sharing with the new Template Manager
will be needed too."

I'd also like to add personas support, for branding/customization.

Please feel free to provide me with feedback.

Regards,
Krisztian