file: replace eel_ref_str with GRefString
glib now offers a reference counted api. This allows us to replace a lot of legacy code and get rid of eel_ref_str. GRefString  is almost a drop-in replacement.
nautilus-file-operations.c was indirectly depending upon eel/eel-string.h from nautilus-file-private.h and thus a new include was added.
This bumps the minimum version of glib to 2.58
Close #781 (closed)