GVariant: Add g_variant_parse_error_quark()

Most GErrors, such as GSomethingError, have a function to get
their quark that looks like g_something_error_quark(),
so bindings (such as gtkmm) would expect GVariantParseError
to have g_variant_parse_error_quark(). Instead this had
g_variant_parser_get_error_quark().
This deprecates the old function and adds the correct one,
making life easier for gtkmm (and maybe others).

https://bugzilla.gnome.org/show_bug.cgi?id=708212