[A11y] Audio events should indicate when they are selected and deselected

[Joanmarie]

In order to interact with an audio event via the keyboard, one moves focus to it and then presses Return. At that point, the audio event becomes shaded to visually indicate it is selected. Pressing Return again causes this shading to be removed, visually indicating the event is no longer selected. These changes in state do not seem to be reported via AT-SPI. As a result, there is no way for a screen reader such as Orca to present this information to a user who is blind.

Please:

1. Emit an object:state-changed:selected event (detail1 being a boolean indicating the new selection state) when the above takes place.

2. Add/remove ATK_STATE_SELECTED from the AtkObject as appropriate. (Note, this may happen as a side effect of doing item 1, but I'm not sure about that. You can use Accerciser to view the states.)