Allow a client to receive notifications through AtspiEventListeners. This
involves special casing, since the event is generated by the screen reader,
not the GUI, and is, thus, handled by a different DBus interface, but, from
the client's perspective, it looks like a normal AT-SPI event.
Also renamed atspi_text_notify_read_position to
atspi_text_notify_reading_position and fixed a few warnings.