Organize nodes by hierarchy (e.g., pretreatment, primary, secondary, etc.)

[fletchapin]

Is your feature request related to a problem? Please describe.
The node sidebar is listed in an unordered fashion.

Describe the solution you'd like
It would make sense to organize node classes by a hierarchy like pretreatment, primary, secondary, solids processing, energy recovery, etc.

The option to hide certain categories could be added as well.

Describe alternatives you've considered
The current sidebar works, but is just cluttered.

Additional context