[GSOC]Use Widget Layout for the Start Center Weekly Update 07

Hi all!

Since my last update I got a lot of feedback about the new Start Center.
Some of the suggestions I have already implemented, but I'm still working
on them.

I'm also working on adding a toggle button to switch between recent docs
and templates, but I encountered a problem.
Here's a patch with a non-functioning test version:
https://gerrit.libreoffice.org/#/c/5543/
I have two custom widgets in a grid, one for templates and one for recent
docs, but they both render as a narrow white strip. (See the tab for
presentations.) On the last tab, you can see that the custom widget for
templates is working properly.

Some other issues I'm having:

I was asked if I could make the scrolling smoother (it always moves
thumbnails 1 row at once when the scrollbar passes a treshold) in the
RecentDocsView class I made. RecentDocsView inherits from ThumbnailView,
which uses Window::HandleScrollCommand for scrolling. Is it possible to
make it smoother without basically
reimplementing Window::HandleScrollCommand?

In the original Start Center, one could drag and drop files onto it, and
they would open. I noticed that this only worked if one didn't drop the
file on a button, but on the background. After converting to .ui, it only
works if one drops the file on the edge of the window, where there aren't
any widgets covering the background. Is there a way to cover the whole
window with the drag&drop functionality?

I was asked to make the background of RecentDocsView transparent, instead
of white. I checked, and ThumbnailView::Paint draws a rounded rectangle
with it's set background color. I set this color to COL_TRANSPARENT, and
now indeed it seems not to draw this rectangle, but the background of the
custom widget is still a white rectangle. Is this behavior inherited from
Window or Control or something? And is it possible to change it? The
inherited Paint methods don't seem to be drawing anything.

If anyone has any ideas about these issues, please, let me know.

All the best,
Krisztian