Instrument container plugin

Enhancement Summary

1. Add a container plugin capable of loading multiple other instruments (unlimited number).
2. By extension, ability to assign and map user-input key(range) passed to Plugin, to instrument-received key(range), so it can map entire ranges, multiple keys, or an individual key. (C4->D5 of instrument1; D4-D5->C2-C3 of instrument2; C3-D3->B3,B4,B5 of instrument3 etc).
   Simply put: load multiple instruments, assign keyIn to keySent.
3. The individual instruments may have a master volume knob or something similar to the W/D mix in the effect chain.
4. The plugin may have a master ability to put filters, envelopes, stacking and effects, so it behaves like a typical instrument plugin but with the components just being more plugins.
5. Additionally, ability to cycle through GUIs of component instruments would be useful. Especially over the same window.

Justification

Several possible use cases due to ability to load multiple instruments and assign keys.
Most common include:

1. Drum pad / sound box functionality

• Drumkits can be loaded as AudioFileProcessor instances for samples, but also Kicker, 3xOsc... no restriction on your percussive choices, as opposed to a typical idea of a drum kit that is limited to loading samples.
• Since it can synthesize sounds too, it can function like an sfx Box of sorts. Highly convenient.

2. Layering sounds and instruments

• Multiple instruments playing the same sound. Useful from a sound design perspective.

Mockup

Note:

• Previously mentioned: https://github.com/LMMS/lmms/issues/1460#issuecomment-67534445 but 9 years ago, when not stable enough a core.
• Similar issue: https://github.com/LMMS/lmms/pull/4075 which brings up sample-based drumpad.
• Related issue: https://github.com/LMMS/lmms/issues/793 which mentions daisy-chaining and requests a specialized MIDI-Out plugin.
• Related issue: https://github.com/LMMS/lmms/issues/443 which asks for instrument layering. (I'm sure there's more, but I cannot find them.)
• Similar issue: https://github.com/LMMS/lmms/issues/6118 closed in favor of this one.
• Implementation of envelopes in the plugin could be tricky and ugly. Possible alternatives include showing the same tab of the component instrument plugin if (see point 5 of summary) when navigating through instruments, allowing users to set similar envelopes for each. Also, a set-global-envelope button for taking one instrument's parameter (volume/cutoff/reso) envelope and setting the same parameter envelope to those values. Sounds painful.

No source files, tests, or entry points are named. Start by reviewing linked issues #1460, #793, #443, #6118 and pull request #4075; the work is done only after the broad feature scope and implementation requirements are narrowed and agreed.