[BUG][FR] Various suggestions to improve Lua

[jameshball]

Various suggestions from @zippy731 below, thank you so much!

If you're updating the UI for fx, here's wishlist form factor that would be great:
Base:

• 3 custom .lua FX slots
• Each with access to the common slider_a/slider_b bank
• FX slots have access to the same env variables (frequency, sample_rate, steps, slider_* values ) as the main .lua synth. In case where multiple MIDI voices are being used, frequency sh/b just voice 1.
• slider_a/slider_b panel should be visible/openable even if synth source is NOT .lua file

Nice-To-Haves / Later

• Custom FX slots reorderable like regular FX
• Able to update the labels on the FX slots and the sliders, so we could drag them around without losing track.
• Dedicated sliders within each custom fx slot (slider_fx1a, fx1b, fx2a, fx2b, etc)
• Access to frequencies or MIDI values for the other MIDI voices, for use as keyswitches or additional processing on chords etc
• console or other space to view/print variables