Improvements to Mp4 64bit data length code

[IJabz]

Because 64bit data lengths as sometimes used by Mdat atom is not stored stored in the header but the subsequent data it does not fit nicely into our 'read header to get data length, and then store in header object' process,

This means that currently Mp4AtomTree does not actually display the data length for 64bit data lengths. We have only fully implemented for MDat lengths, in theory other fields may make use of 64bit data lengths even if the data could fit into 32bit value. Even if could store in header we would need to change header data length field from into to long and this could have other consequences.