Provide accessor method on Scene to get the Camera component that has been specified as Default

Provides an accessor method to get a "Default Camera" doesn't provide any functional impact but reduces the requirements to pass around or search for the Camera similar to the way scene is registered with Script for lookups.

Angelscript bindings included, lua not as I have no idea if the forward decl works with lua and can't register in a different order like you can in angelscript.