Revert "a11y: Remove AtkSelection implementation from combobox"

This reverts commit 7cecc8e524.

It is impossible to use the selection object of the menu while it is
collapsed (collapsed menus deselect everything for a start), so even
though the original patch was correct, the followup issues are too big
to solve this quickly to a release.

https://bugzilla.gnome.org/show_bug.cgi?id=707786

testsuite/a11y/pickers.txt

@@ -58,6 +58,7 @@ window1
       <AtkAction>
       action 0 name: press
       action 0 description: Presses the combobox
+      <AtkSelection>
       unnamed-GtkMenuAccessible-0
         "menu"
         parent: button1