When loading a Session add the Session patchfiles directory to the
MidiPatchManager search path and only process/parse the files for that
directory rather than refreshing/reparsing all the files. Similarly for unload,
just unload the devices that are from the Session specific midnam files instead
of removing the path and refreshing/reparsing all the files.
This will not remove the "system" midnam files as they are always added first
and duplicates from the session patchfiles directory are ignored.
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