Add Matrix layout

So I added a simple layout that arranges windows in a matrix of equally sized cells. I find this useful when I need to open lots of windows (i.e. logging into a cluster of machines using ssh).

I just not realize that this is sort of similar to RatioTile in functionality. The navigation more similar to the Stack layout and the implementation might be less complex.

If it's to similar to RatioTile it might not fit. I think it's kind of nice.