In practice there is always at least one profile, and I
don't think there will ever be cards without profiles.
Therefore, I added assertions to pa_card_new() stating that
the card new data must always contain at least one profile.
Now a lot of code can be simplified, because it's guaranteed
that the profiles hashmap and the active_profile field are
always non-NULL.
* Make the dbus object constructors take a pa_dbusiface_core pointer
as an argument. Remove the path_prefix argument.
* Expose the core object path as a constant in protocol-dbus.h.
* Move the core interface name constant from iface-core.h to
protocol-dbus.h.
