Photoviewer: Allow using the Android back button to go back.

Don't show back button on Android, as the system back button can
now be used.

Task-number: QTBUG-38122
Change-Id: Ia3a677f4626eaf34e23dfcc7997e4b850d8b7020
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>
Fix type of AnimationController::animation in documentation.

Change-Id: I3122ba1d7e9f1c396bef347cd1fd7df8ab32ff47
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>