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/combos.txt

@@ -69,6 +69,7 @@ window1
       action 0 name: press
       action 0 description: Presses the combobox
       action 0 keybinding: <Alt>c
+      <AtkSelection>
       unnamed-GtkMenuAccessible-0
         "menu"
         parent: combo1
@@ -91,6 +92,7 @@ window1
       <AtkAction>
       action 0 name: press
       action 0 description: Presses the combobox
+      <AtkSelection>
       unnamed-GtkMenuAccessible-1
         "menu"
         parent: combo2
@@ -154,6 +156,7 @@ window1
       <AtkAction>
       action 0 name: press
       action 0 description: Presses the combobox
+      <AtkSelection>
       unnamed-GtkMenuAccessible-2
         "menu"
         parent: combo3
@@ -176,6 +179,7 @@ window1
       <AtkAction>
       action 0 name: press
       action 0 description: Presses the combobox
+      <AtkSelection>
       unnamed-GtkMenuAccessible-3
         "menu"
         parent: combo4

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