Added Count to Opacity converter and added null checking to StringToImageBrushConverter.  Also finished Music Pages

Added Count to Opacity converter so in the semantic zoom the letter will be faded if there are no items in the group so the user can visualise which groups have items easily.

Added null checking to StringToImageBrushConverter because it was breaking if the url was null or empty string. 

Finished the album, artist and all music page. Just needs styling now