Add a menu entry to allow editing the Score file in text editor

Fixes #11 (closed)

I removed the pan.ssh.ui.h file as the same can be achieved by the use of #ifdef - it saved me from having to put the same change in two places and I'm lazy!

This basically adds an 'Edit Score File' entry under the 'Edit' menu which will fire off your editor of choice to edit the score file.

As part of that I lifted out the code to fire off the editor and made it into a separate class, and ensured that you cannot fire off 2 editors for the same file (mail or score file) at the same time.