Rework how nodes are sorted

[RunDevelopment]

Changes:

• Categories and node groups have IDs now. No more "name == id". The value for those ids is the name though. We can change that if needed though.
• Node groups are now a datatype in the frontend too. This means that we can add more metadata to change their appearance/behavior in the future.
• Refactored some frontend code.

The basic idea now is that the backend declares the order of categories, node groups, and some nodes (order), but it does not enforce it. So nodes from /nodes may be in any order. The frontend is responsible for applying that order. It currently does via the sortNodes method. Basically, the first thing the frontend does when getting the response from /nodes is to sort them. Not sure whether the backend should just do that as well.