Implement multiple mouse click detection.

Fixes the ASSERT we were seeing on double clicks and
implements detection for triple, quadruple etc. clicks.

Double mouse clicks forwarded from QMouseEvent are swallowed.

Change-Id: I9be83f809805ab3dea7a508b648046238ce36a90
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>