(ImageSize) Add natural image size handling

(ImageSize) Add natural image size handling

[Issue#]       N/A
[Problem]      Need a way of switching actor between natural image size and
constrained, animated or set size.
[Cause]        Natural image size not handled correctly
[Solution]     Add mechanism to switch to natural size, (using existing
Actor::SetSize() will override this)
[Verification] Build Repo

Change-Id: Id4641d87bc8de68e613a0e10d8dddf26cddae9fb
Signed-off-by: David Steele <david.steele@partner.samsung.com>