Feature/model parameter overrides

[richardtallent]

As discussed in Discord, this allows users to select a preset for any model or LoRA. The preset is added when the model/LoRA are selected. These are stored as user settings.

For models, the preset is unselected (if not applied already) if the user clicks another model that has a different preset.

Documentation is included.

This adds two optional user settings as well:

• Whether to auto-apply presets when selecting a model/LoRA rather than just selecting the corresponding preset.
• Whether to ignore LoRA presets for LoRA loaded through a model's preset, if the LoRA is loaded with a weight of 0 (so the model can effectively disable the LoRA).

This is my first deep trip into the code, so please let me know if I've accidentally re-implemented utility functions, etc. that already exist. I've tried to follow the conventions and style of this repo as much as I can, and to minimize the "blast radius" of the new code.