Move non-blueprint state out of the blueprint

[jleibs]

As of #2010 the entirety of the blueprint is serialized and then reloaded on every frame. This means that anything marked serde(skip) ends up being reset on every frame. This is related to some rendering helper state, some of the hover selection logic pieces, etc. This behavior complicates the logic of saving/restoring blueprint and clouds the understanding what things are for configuration vs what things are for operation.

All of this needs to move to a different runtime-persistent storage which is used in conjunction with the blueprint to drive the layout and behavior of the views.

[emilk]

Related:

• Plan to make Blueprint ergonomic code-wise in the Viewer #2142