RFE: Tremolo and TremoloSingle object support

[ndickson]

It would be awesome if there was support for the Tremolo.ms (for tremolos between two notes / two chords) and TremoloSingle.ms User objects from Noteworthy, such as in bar 1 and bar 124, respectively, in the attached file. WieniawskiViolinConcerto2_3_Cello_2ndViolins.txt

In Noteworthy, the Tremolo.ms object with a 16th-note tremolo (2 beams) between two notes for 2 beats is represented like (bar 1):

|RestChord|Dur:4th,Slur|Opts:Stem=Down,Muted,HideRest|Dur2:Half,Slur|Pos2:#1
|User|Tremolo.ms|Pos:0|Beams:2|Style:3
|RestChord|Dur:4th|Opts:Stem=Down,Muted,HideRest|Dur2:Half|Pos2:-1

For alternating between two chords in the same manner, it's like (from the 1st Violin part, not attached):

|RestChord|Dur:4th,Slur|Opts:Stem=Up,Muted,HideRest|Dur2:Half|Pos2:-1,5
|User|Tremolo.ms|Pos:0|Beams:2|Style:3
|RestChord|Dur:4th|Opts:Stem=Up,Muted,HideRest|Dur2:Half|Pos2:-1,6

The print style is a bit different for alternating between two notes for less than 2, or 4 or more beats, e.g. this example of alternating between two notes for 1 beat, but tripleted, so I made it (with a lot of fiddling) display as dotted quarters with two lines, not connected to the stems, between them (from the Viola part, not attached):

|RestChord|Dur:8th,Slur|Opts:Stem=Up,Crescendo,Muted,HideRest|Dur2:4th,Dotted,Slur|Pos2:0
|User|Tremolo.ms|Pos:0|TripletPlayback:Y|Beams:2
|RestChord|Dur:8th,Slur|Opts:Stem=Up,Crescendo,Muted,HideRest|Dur2:4th,Dotted,Slur|Pos2:2

I think the Beams option might default to 3 if it's not present, and it won't show up in the text if it's set to 3 in the UI, so that may need to be handled specially.

For a single tremolo, it's a bit simpler, since it doesn't rely on RestChord duration quirks. This one is a 3-beam tremolo (bar 124):

|User|TremoloSingle.ms|Pos:-3
|Note|Dur:Half|Pos:-2

There can, however be a vertical offset and a different number of beams for a single tremolo too. This one has a single beam and is moved 1 unit toward the notes along the stem, i.e. 1 unit opposite the stem direction (from the Viola part):

|User|TremoloSingle.ms|Pos:3|Beams:1|Offset:1
|Chord|Dur:Half|Pos:1,6

It also supports the tripleted option, but that doesn't affect display. There's apparently some corresponding thing in MusicXML, though they don't explain it very clearly. Note that the Pos value for the Tremolo and TremoloSingle objects doesn't actually affect their display, so you can ignore it. Unlike with the Hairpin object, it only specifies the location of the anchor in the editor in this case.

Single and double-note tremolos in MusicXML seem to both be controlled using the tremolo tag described here, though they only give a single-note example:

<note default-x="156">
  <pitch>
    <step>F</step>
    <octave>4</octave>
  </pitch>
  <duration>2</duration>
  <voice>1</voice>
  <type>quarter</type>
  <stem default-y="0">up</stem>
  <notations>
    <ornaments>
      <tremolo default-x="6" default-y="-27" type="single">3</tremolo>
    </ornaments>
  </notations>
</note>