Customization is totally missing

Context

Currently, people have no easy way to customize anything about Gelly. There are several reasons for that:

No configuration infrastructure
- As in, we really just have tons of sources of truth for configuration which are scattered all over the code base
  - Fluid simulation module (IFluidSimulation)
  - Fluid renderer module (SplattingRenderer::Settings)
  - Composite module (PipelineConfig)
  - Lua module (GELLY_SIM_TIMESCALE, GELLY_ENABLE_SIM)
Level of abstraction has not yet been decided
- i.e: should the graphics be organized like Roblox/Fortnite with "qualities" or like GW2 with specific options and sliders for every internal setting
- i.e: should we create a set of fluid parameters like: smoothness, stickiness, thickness? or should we expose direct access to FleX parameters like fluid rest distance, viscosity, cohesion, adhesion, etc.
There is no UX design for configuration
- There's no UX design of any kind for a sort of central control panel, like GWater2.
- Ideation and brainstorming has to be done first
- This is decoupled from the above settings

Create a central facade for use in gelly-gmod which controls all necessary non-ephemeral settings
- Should allow access to a central setting structure
- Said structure should have get/set functions (will be serialized by the addon)
Create a UI for managing the new settings