[orca-list] Speak selection command now bound

[Joanmarie Diggs <jdiggs igalia com>]

Hey all.

I've now bound the command to speak the current selection. Given there
wasn't much feedback on this, I went with Orca Shift Up. This appears to
be consistent with NVDA. I hope this works for you.

I've also expanded the functionality of this command to present the
selected stuff (text in a text object, cells in a spreadsheet, icons in
an icon view, etc.).

--joanie