WIP-MEI: Fix the mid-measure clef issue

Previously, staffFromElement() pulled Clef objects out of the Voice and into the
Measure directly. There was a good reason for this I guess, but it doesn't seem
to exist any more. Now that Clef objects are left in their Voice, they also stay
at their imported position.
WIP-MEI: Robuster detection in converter.Converter.getFormatFromFileExtension()

On POSIX systems (i.e., everything but Windows) the "file" command is used to
help determine whether the MIME-type and file extension both match a format
known to music21. This allows more reliable filetype detection, more consistent
error messages with unsupported types, and greater resilience against
maliciously-constructed files.
WIP-MEI: Robuster encoding detection in SubConverter.parseFile()

On POSIX systems (i.e., everything but Windows) the "file" command is used to
help determine the character encoding of a file about to be imported, ignoring
any declared encoding within the file. On non-POSIX systems, parseFile() tries
first with UTF-8 then UTF-16 before allowing a failure. UTF-16 is used by the
Sibelius-to-MEI exporter, so it will appear relatively more often on Windows(!)