fix on StringSize for WPF & WPF sample compiles again

Hi Frank!

I just fixed IFontMetrics.StringWidth() for WPF.
I don't know how you got that working on your side, but maybe Silverligth was just another story.
In WPF it is necessary to have that TextBox - the one that is misused to just retrieve the text width - to have that TextBox somehow rooted within a canvas (or maybe somewhere else also possible). 

So now we would just create one TextBox per canvas, put that in the canvas we get from the user, hide it, and use it for all text meassurements for that XamlGraphics...

Hope you like it. (Whitespaces in the VisualStudio are fine) Sorry for that!

Thanks for this project! It is fun!

See you later,
Sebastian