VDP connection as a file handle

[stevesims]

Building on #80 and #81 the basic suggestion made in there is that the VDP connection could be a "file handle" in much the same way that a UART1 can be considered a file handle

By treating the VDP connection as a file handle, this potentially gives us a simple mechanism for MOS to receive arbitrary data from the VDP using an API that would be consistent with other APIs we already have in place.