Introduce new visual transform property : EXTRA_SIZE

Introduce new visual transform property : EXTRA_SIZE

The EXTRA_SIZE can increase/decrease the computed visual size.

The property can be used when a user want to set a visual size
as a combined value of `absolute` and `relative`: ControlSize * a + b

For example, when a user want to set a visual size to (ControlSize * 2 + 10),
The transform map will be,

transformMap.Add( Transform::Property::SIZE, Vector2( 2.0f, 2.0f ) )
            .Add( Transform::Property::SIZE_POLICY, Vector2( Transform::Policy::Relative, Transform::Policy::Relative ) )
            .Add( Transform::Property::EXTRA_SIZE, Vector2( 10.0f, 10.0f ) );

Change-Id: Ibd1b7667a96900f216f7a9a14ea6813636161b48
Signed-off-by: Jiyun Yang <ji.yang@samsung.com>