Obsoletes UmbracoFile with new UmbracoMediaFile in the Core assembly and wraps the legacy class to use the new one. Improves performance of UmbracoMediaFile by ensuring any IO is lazy loaded and then saved to a local property so it doesn't need to re-read the file for things like length/size.

Conflicts:
	src/Umbraco.Core/Umbraco.Core.csproj