The old GIMP_CONFIG_INSTALL_PROP_FOO() have the problem of always
passing NULL as the GParamSpec's "nick". I have no clue what we were
thinking back then, but this has always been a major design flaw
because (among other problems) it makes it impossible to fully
auto-generate GUIs based on properties.

Added GIMP_CONFIG_PROP_FOO() macros which do have a "nick" parameter,
will deprecate the old macros as soon as everything is ported.