Support audio processing with more than 2 channels

Some plugins require more than 2 channels for its audio input/output. Here are some common use-cases:
- Stereo sidechain compressor: requires 2 stereo inputs
- Drum kits: some drum kit VSTs can use multiple outputs for multiple samples, for examples, stereo out 1 for kick, stereo out 2 for snare

However, there are some difficulties which come from the internal structure of the LMMS core:
- LMMS uses interleaved stereo formats while external plugins like VST, LADSPA, Zyn, and LV2 expect non-interleaved format.
For audio outputs and file outputs, the interleaved form might be beneficial. However, I think it's not for internal processing.

Features that can be implemented:
- Multi-channel matrix effect, as an extension of the current stereo matrix plugin
- Better support for sidechain compressor or vocoder plugin, see #3817 and #4337
- Native drum sampler(#4075) with per-sample effect mixing

No files, tests, or specific entry points are named. Start by mapping LMMS core audio processing and the plugin interfaces mentioned, then review issues #3817, #4337, and #4075; done would require a decided multi-channel design and working support for the selected use cases.

Written by the indexing model from the issue text.