app: change GimpImageFile and GimpDocumentList to speak GFile
app: port all file_open() and file_save() functions to GFile
app: port file_open_from_command_line() to GFile

Use g_file_new_for_commandline_arg() and remove
file_utils_any_to_file().
app: change the image's various URIs to GFiles

which gets rid of a lot of temporarily added GFile code again because
we don't need to convert between URI and GFile any longer.