Suggested Enhancement: Allow GTK option

[mikegilchrist]

I've tried passing the argument -ui graphic to get the gtk version of unison, but it doesn't work for me. As a work around I've modified my local code, replacing new_cmd = ["unison"] + cmd with new_cmd = ["unison-gtk"] + cmd, but suspect it could be incorporated into the code in a way similar to how unison uses unison-gtk for the GUI, i.e. unison-gitignore-gtk