GVariant: add way to print context of parse errors

This was a feature intended from the very beginning that somehow never
got written.  It's a way to replace these sort of error messages out of
the GVariant parser:

  1-2,10-15:unable to find a common type

with something in the style of the Vala compiler:

  unable to find a common type:
    [1, 2, 3, 'str']
     ^        ^^^^^

https://bugzilla.gnome.org/show_bug.cgi?id=715028

docs/reference/glib/glib-sections.txt

@@ -3229,6 +3229,7 @@ G_VARIANT_PARSE_ERROR
 g_variant_parse
 g_variant_new_parsed_va
 g_variant_new_parsed
+g_variant_parse_error_print_context
 
 <SUBSECTION Private>
 g_variant_parse_error_quark