General cleanup of RTP-MIDI dissector.

The biggest changes involved:
1. Use the "bounds checking" of proto_tree_add_xxx, instead of doing it manually. There are still places where it's done "manually", but they have been drastically reduced.  Someone with more understanding of the protocol/dissector would need to take a look at them.
2. proto_tree_add_text -> proto_tree_add_subtree[_format]
3. Use proto_tree_add_bitmask when appropriate.

Change-Id: Iddbd2aadf5fd27f2cf9ba63873eb59dbd93b6394
Reviewed-on: https://code.wireshark.org/review/4039
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>