So far, the PLY exporter was only able to write ASCII-encoded.ply files.
This commit extends the PLY exporter such that it is now able to also
write binary files, both little-endian and big-endian. The user can
choose the preferred format variation using a new drop-down.
By default, "Text" is selected, such that the current behavior stays the
default behavior.

This change was mostly motivated since I am currently writing my own
stand-alone .ply tool and it is easier to read raw binary data than to
do string parsing.
Not sure if this change is valuable for any other use case, too, but
I thought to submit it nevertheless, maybe it turns out to be useful for
someone else.