Merge the function pointer get_func() into to_dbus_fcn().

Previously, get_func() as handled separately from to_dbus_fnc()
(formerly synth_func()). The notion was that synth-func would syntetize
properties that are D-Bus only. But that distinction does not seem
very helpful to me.

Instaed, we want to convert a property to D-Bus. Period. The
implementation should be handled uniformly. Hence, now that is
all done by property_to_dbus().

Note that property_to_dbus() is also called as default implementation
for compare-property. At least, for properties that are backed by a
GObject property.