stylepropertis: Use set_by_property() in set_valist()

stylepropertis: Use set_by_property() in set_valist()

... instead of duplicating code. This causes an extra g_value_copy().
If that turns out to be a performance issue, we can invent something
that handles this (like passing a gboolean take_value).

The reason for this duplication deletion is that we want to complicate
the setting code to handle shorthands by unpacking them and storing the
separate values.