[GSOC]Use Widget Layout for the Start Center Weekly Update 02

Hi all!

Last week I have:

- Been busy with my final exam, so I had less time for GSoC. Thankfully, I
passed.
- Been trying to figure out why nothing gets drawn after changing the
buttons to .ui buttons.
Currently BackingWindow inherits from Window and VclBuilderContainer, and
everything else is done like how the wiki page about .ui conversion says. (
https://wiki.documentfoundation.org/Development/WidgetLayout)

I got rid of all the manual formatting code, then put it back, then removed
it again, I cleaned the code to the bare bones, and all I managed to
achieve is that the window is now drawn in white instead of light grey.
Kendy suggested that maybe it's because I haven't redefined GetOptimalSize
but adding it didn't seem to help.
I printed the position/size of the gui elements from the loaded .ui file,
and both of them were (0,0) for all of them. I thought this was an error
but I got similar results for other .uis that are drawn correctly.

I plan on looking more at Dialog and TabPage to figure out what is it
exactly what I'm not doing that makes things draw. (It's probably magic and
pixie dust.)

In my last report I mentioned that the dtor for Window gets called twice,
I'm not sure about this anymore, it might be something that happens because
the conversion to .ui is only partial and there are resource based bits in
there. I want to focus on getting the .ui buttons to work right, and see if
this problem sorts itself out.

If anyone has suggestions to what I'm missing I'd be very interested to
hear them!

All the best,
Krisztian