New app which generates png screengrabs from .gfr scripts

This app is to be used during the documentation build - it loads a .gfr script in a gui session then saves a snapshot png to the specified location.

Contains two lines are are know to be hacky (75 and 119) but is currently functional. 119 will hopefully be substituted for a more robust gaffer api call that would additionally support grabbing any widget rather than just the main window, once that call is implemented.

Any suggestions for improvements welcome