nautilus-file-operations: Use mount name if available

Currently custom_basename_to_string uses the base name or display name
of the files for the copying and moving dialog.
In the cases of unlabeled removable drives, the name is a code, showing
in the dialog a not very friendly name.
gtkplacessidebar and nautilus-pathbar uses the mount name retrieved by
g_mount_name if available.
Use that fir the operations dialogs as well to display a more friendly
name to the user and to be consistent with the sidebar and pathbar name.

https://bugzilla.gnome.org/show_bug.cgi?id=738087