gio: New API for GFile from remote commandline arg

gio: New API for GFile from remote commandline arg

Add a pair of new APIs: one to GFile to create a new file from a
commandline arg relative to a given cwd and one to
GApplicationCommandLine to create a GFile from an arg, relative to the
cwd of the invoking commandline.

https://bugzilla.gnome.org/show_bug.cgi?id=689037