Add Calendar::pressAndHold(date) signal

[ChangeLog][Calendar] Added pressAndHold(date) signal that is emitted
when the user presses and holds a valid date.

Change-Id: Ie880a3ed827332d35fabc76cd6a09eebc7685eb7
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Fix Calendar::clicked() signal emission

Emit only when releasing over the same date than was pressed.

Change-Id: I7a8d4d6e3871231ae2589d565a75d9b641b649ac
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>