ResourceImage/Image split

ResourceImage/Image split

Move all of the features relevant to images loaded via URL into a derived class:
-	Querying whether the image has loaded (GetLoadingState)
-	Querying the URL (GetFilename renamed to GetUrl)
-	Signal when the loading succeeds or fails (LoadingFinishedSignal)
-	The Reload() feature
-	The LoadPolicy feature

This also enables the derived type to be distinguished from image types such as
FrameBufferImage, which are not compatible with certain features (e.g. Atlas)

Change-Id: I708d8df6c8ff32067143c022100d6b61b1405ee5