Refactor JavaScriptDialogManagerQt to support a Qt Quick dialogs
friendly approach.
Qt Quick dialogs are still missing a prompt, so we use a "handmade"
one. This should be solved before 5.3 though.

Change-Id: I965df66837b2e81d6e4618a8da1167a37661c26e
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>