Minor changes to ATK signals

["Padraig O'Briain" <Padraig Obriain Sun COM>]

I would like to make the following changes to signals in ATK despite the API 
freeze.

The definition of the signals is inconsistent with their use; i.e. the signals 
are emitted only by implementors of the interface.

1) Remove the signals "selection-changed" and "model-changed" from AtkObject.

2) Add the signal "selection-changed to the interface AtkSelection.

3) Add the signal "text-selection-changed" to the interface AtkText

4) Add the signal "model-changed" to the interface AtkTable

Padraig