Remove filename/URI API from GtkFileChooser

Remove filename/URI API from GtkFileChooser

GtkFileChooser's API predates GIO by a few years, so it started off with
filenames and URI as character arrays. After introducing GIO as a
dependency, the API included GFile-based entry points.

It's much more appropriate to use GFile everywhere, as we want to
encourage people to use GIO instead of passing random bytes to low level
POSIX API.

See: #2455