Make indexed database follow Qt web settings if enabled

Forwards the websettings for offline database storage which so far
has been used for web SQL database to indexed database.

Note that this does not add new API and does not make it possible to
control web sql database and indexed database separately.

The patch also adds a config test for leveldb which indexed db requires.

Change-Id: I8808356c9910848c43c5f47e6279a1c632f17ad1
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>