TableView: let delegates handle mouse events

Includes auto tests.

Task-number: QTBUG-31206
Change-Id: I683363f70f7e360cc70f84fd460cbde5e70a61af
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>