libgimpconfig: Ensure G_PARAM_STATIC_STRINGS

The GIMP_CONFIG_PROP_* macros do not automatically set G_PARAM_STATIC_STRINGS, which means each property that uses such a macro does not have this flag set by default (and almost none set it themselves). By adding G_PARAM_STATIC_STRINGS to GIMP_CONFIG_PARAM_FLAGS, we can prevent this from happening, which should lead to some unnecessary string copies being prevented.