libobs: Deprecate v1 of obs_properties_add_button

With v1 of this function, it's unclear where exactly the data pointer
comes from or what it is. In fact, this is not determined by libobs, but
the consumer. libobs assumes that the caller of
obs_property_button_clicked passes an obs_context_data pointer, and then
passes the data pointer of that obs_context_data as the data pointer to
the callback.
In OBS Studio, this is always the private data of the associated object.
However, this assumes that there even is such an object (source/encoder/
etc), even though properties are meant to be free-standing. This is not
just philosophical, because with obs_get_source_properties you can
actually get an obs_properties_t that isn't associated with any specific
source, at which point you have no idea what the data pointer will be.

For this reason, obs_properties_add_button v1 needs to go.
obs_properties_add_button2 can be used as a drop-in replacement.
With v2, it's well-defined that the pointer you're passing as priv is
the pointer you get back in the callback as data. If you don't care
about it, simply pass NULL/nullptr.

Once v1 is removed in the future, obs_property_button_clicked should be
replaced with a variant that doesn't take a second argument, as that
argument will no longer be used anywhere.