Remove the `PROXIMITY_LIMIT` constant in favour of an adjustable GUI slider under the "Master" panel

[mitchmindtree]

This value can be useful for tuning the range of speakers that a sound channel position may reach as this can significantly affect CPU performance in larger exhibitions with 100+ speakers & 100+ sounds.

[mitchmindtree]

To implement:

• store a copy of the proximity_limit in the project's config struct. This will allow it to be automatically loaded from the project's config.toml file at startup and saved on close.
• add slider widget to master panel for updating this value
• also add proximity_limit field to audio output model
• send proximity_limit update to audio output model on slider update