We have certain artificial properties that not only depend on one
property alone or that depend on a property in another(!) setting.

For that, we have synth_func.

Other than that, synth_func and get_func are really fundamentally
similar and should be merged. That is because the distinction whether a
property value is "synthetized" or just based on a plain property is
minor. It's better to have the general concept of "convert property to
GVariant" in one form only.

Note that compare_property() is by default implemented based
on get_func. Hence, if get_func and synth_func get merged,
compare_property() will also require access to the NMConnection.

Also it makes some sense: some properties are artificial and actually
stored in "another" setting of the connection. But still, the property
descriptor for the property is in this setting. The example is the
"bond.interface-name" which only exists on D-Bus. It's stored as
"connection.interface-name".
I don't really like to say "exists on D-Bus only". It's still a valid
property, despite in NMSetting it's stored somehow differently (or not
at all). So, this is also just a regular property for which we have a
property-info vtable.
Does it make sense to compare such properties? Maybe. But the point is that
compare_property() function needs sometimes access to the entire
connection. So add the argument.