Open hex editor on a sysex file

[krevis]

IIRC we can now embed the HexFiend editor into other apps -- that would be far easier than writing our own hex editor.

[krevis]

As of a0baf8d286c5447299ca64abb422913149d0ff10 we're now using Hex Fiend to display hex dumps in SysEx Librarian and MIDI Monitor. Looks a lot nicer.

However, sysex files aren't editable yet. Still need to figure out the exact workflow.

• It would be bad if it were possible to easily destroy your sysex file with just a few clicks or errant keypresses. Make sure it's difficult.
• Don't allow adding bytes > 0x7F which would make the sysex invalid.
• Allow both insert and replace modes.
• Easily allow copy/paste of whole messages in the file. And rearranging them too.