Enable HTML5 Indexed Database API

Enable the HTML5 indexed database API which is now a W3C candidate
recommendation and supercedes WebSQL.

[ChangeLog][QtWebKit][New Features] Indexed Database API (not available on windows)

Change-Id: I924ba9cc2218658822380577dbd23ab010ef1f76
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>