Add PreloadingProxyElement to ExamplePlayer, to demonstrate how a proxy can preload a VideoElement within a SerialElement and make it seekable prior to the playhead reaching that child of the SerialElement. Helps demonstrate a solution for the issue raised in FM-952, in which it's not possible to seek across subclips within a SerialElement. Modified existing AsynchLoadingProxyElement so that it could serve as a base class for the new proxy.