[WIP] DataGrid Style - Extends #446, Resolves #377

I've rebased #446 and have integrated it into the new demo app

A couple of little visual things to sort out:

 - [ ] demo, both -  should the header borders be thinner/lighter?
 - [ ] demo, light theme - second grid has first column highlighed in light blue - why?
 - [ ] demo - second grid should be disabled to show differences (?) 
 - [ ] demo, dark theme - second grid has first column highlighed in dark blue - why?

Also has 075fd42 which resolves the scroll viewer being visible for the AnimatedSingleRowTabControl control.

Test it out, let me know your thoughts.