User assignable multiple channels per staff

Proposed feature:
  A dialogue permitting user-assignable channels for any staff.

Proposed locations:
  a) ellipsis buttons beside entries in the Instruments dialogue
  b) a tab in the Staff Text Properties dialogue

Rationale:

Directions for changing articlation and/or timbre need to be executed during playback. Some examples:

• woodwinds a due vs. solo
• French Horns stopped or bells up
• trumpets with different mutes
• strings legato vs. détaché, or con sord., or harmonics
• piano una corda

At present, only a few instruments are assigned more than one channel to deal with articulation/timbre changes during playback (e.g. trumpets, strings). For all other instruments, such changes require setting up hidden staves, or changing instruments mid-staff with Instrument from the Text Palette.

Hidden staves in a large orchestral score are a problem because revealing even just a few can cause the score to exceed the page length, plus there's the headache of silencing notes on the non-hidden staff.

Changing instruments mid-staff with Instrument adds a new entry to the mixer every time the instrument is changed. E.g., a piano score with six changes from the tre corde “instrument” to the una corda “instrument” results in twelve entries in the Mixer, each of which may have to have its volume changed in the Mixer because the volume of each new instrument added to a staff uses the volume in effect when the instrument is added.

It makes very much more sense to add channels to staves to handle the different articulations and timbres required for playback than to “change instruments” or add hidden staves. For example, a user could add the Staff Text direction “a due” to the oboe staff, right click on Staff Text Properties, and select the a due channel, which has been setup in the Mixer to use an “oboe section” soundfont.

There are three distinct advantages to this:

1. Staff Text directions can be executed during playback.
2. There's only one Mixer entry for each alternative articulation/timbre.
3. The Mixer entries for alternative articulations/timbres associated with any given staff are grouped together rather than appearing on-the-fy at the bottom of the Mixer list.

The only way currently to assign multiple channels to a staff is to edit the .mscx file and add them there (along with helpful names like “pavillons en l'air”)—a method I use routinely and which drastically improves the overall realism of playback.

My proposal is that users be given a dialogue from within MuseScore that allows adding any number of named channels to a staff, thereby circumventing the need to edit the .mscx file. The dialogue would simply ask for the number of channels, with text fields for each channel so they can be named sensibly.

Without knowing the internals, I believe such a dialogue can be implemented easily since it merely affects what's written out to files rather than requiring any new code in the core binary.