Accessibility: Table interface

This is used to represent tables, lists and trees to screen readers.
There are some properties still not implemented, notably selection which
is also not available in blink.

Change-Id: I57053881445e39a4967a1aabd85a594271402f63
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>