Initialize the "id" attribute when adding particles to the state.
Use a readonly flag for attributes and a second variant of the Data
RNA access types to prevent writing over immutable attributes ("id" in
particular).
Add position attribute to particles by default.
Basic display feature for particles. This is organized as a list of
NParticleDisplay in the modifier for now. Each NParticleDisplay adds
a visualization element of some type in the viewport, which will allow
combining elements for custom attributes with standard display modes
for particles.