Add two features: test_forces running from GUI, and calling gerbv to preview gerber

[colinoflynn]

Two changes

1. Added a button to create the test_forces file, which can be handy during setup. Have added to end of list as don't want to click it by accident.
2. Added a button to display the input gerber file in gerbv (tested on Windows)

[pmonta]

Thanks---sorry to get back to you so late; the holidays were crazy busy. This all looks fine; before I merge it I just have one question: is there a reason for the "os.name=='nt'" test in show_gerber()? The code looks platform-independent, so it might be reasonable to omit the test and check for the existence of gerbv and pstoedit on every platform, not just Windows.

[colinoflynn]

Oops... copy-paste error I think! Let me try and push a fix...

[colinoflynn]

Test on Windows this seems to work fine anyway, but should work on Linux too - I don't have this setup on Linux, not sure if you can easily test? If not will try to install the required tools onto my Linux box later.