Add a qset editor

[FrenjaminBanklin]

It's technically possible to save and load a qset in the creator, but loading a custom qset in the player is severely limited - either demo.json would have to be changed, or an alternate demo_dev.json would have to be provided.

In addition to enabling the player to load instance qsets like the creator does, there should also be an interface (either built into the player/creator or as a separate page) to view/adjust those qsets.

[FrenjaminBanklin]

Halfway addressed by #36, but it might be nice to resurrect old DevMateria's tool panel - which among other things had a qset parser that allowed us to alter the JSON directly and inject it into the player.

[FrenjaminBanklin]

See https://codemirror.net/.