avoid double-setting synth ui sliders when setting from mapper

[balintlaczko]

• test throttling
• test debouncing

[balintlaczko]

Implemented debouncing where all synths create their own "debouncer thread" to avoid spamming the sliders with messages to set its value. The slider updates do look less attractive now, since there is a 50 ms lag from the last Probe movement until the sliders update, and also, they won't update while there is a continuous movement. I may experiment with a better tradeoff, but for now this avoids the ui becoming a liability to the interactivity if there are lots of sliders.

Also, every slider unobserves its callback before it is set, so the double-setting loop (where the synth updated the slider when its param buffer was set from a mapper, only to get the param buffer set again once or twice from the slider too).

The new setup also seems to fix the mysterious disconnects from sliders that only happened in the VS Code interactive from time to time. They may have happened because the widget models were overloaded with websocket messages(?).