First efforts on creating a PropertyGrid

So... I was reading again the PR comments on the Monogame repo, and Tom said:

> It is a little more complex... but sort of. The value side of the property grid can be more than just text, could be text + button, a combobox, a check box, etc. Also you have expanding sections which can be rolled up to hide some nested grid items.
> If we get something working well it might be worth contributing it back to XWT. If they had a complete set of controls we could just use it across all platforms.
https://github.com/mono/MonoGame/pull/2441#issuecomment-47991196

So I tried to create a PropertyGrid using only Xwt controls and created a Mockup.

I think it's doing good. Have a look, please.