This change makes it so changes to the presets file are no longer being
written to metadata. Instead, the preset data is read separately
(although it is still possible to override it with metadata if done
manually).
Because presets are being tracked separately, if the associated metadata
key is not set, changes to the `presets/ai.json` file will be applied
immediately without switching the preset choice. Note that `ai:preset`
is still used in metadata to associate a block with a preset.
Additionallly, this introduces a database migration to clear out the
metadata items starting with `ai:` except for `ai:preset`. This will
allow the change to apply to existing blocks in addition to new ones.
I am updating the layout node setup to write to its own wave object.
The existing setup requires me to plumb the layout updates through every
time the tab gets updated, which produces a lot of annoying and
unintuitive design patterns. With this new setup, the tab object doesn't
get written to when the layout changes, only the layout object will get
written to. This prevents collisions when both the tab object and the
layout node object are getting updated, such as when a new block is
added or deleted.
