Allow user defined clefs

This would be easier if users were able to define their own clefs.

My suggestion is the creation of a define clef dialog that would enable the user to select either a clef symbol or possibly an svg file (since they are easily scaled) to represent the clef (I've seen a request to be able to use an N similar to the note input icon as a clef). The user would then define the location of written middle C and any equal octave transpositions only. There is no need for transpositions like major third or perfect fifth and so on.

The interface would be very similar to the key signatures master palette. It would include

• a staff where the clef would be shown
• a single note showing the location of written middle C
• a numeric (scroll?) box that allows the user to move middle C up (negative number for consistency) or down (positive number) one line/space at a time
• a transpose line like at the bottom of Staff Properties that only includes octaves and up/down radio buttons.
• a list of available Clefs and possibly a ??? icon to allow the user to upload a custom clef as an SVG file.

Written middle C: In the case of an instrument like an electric bass, middle C is actually an octave above what it looks like. In the case of the electric bass what I'm calling written middle C is the first ledger line above the staff.