Entity spawn panel preview icon entities aren't paused and don't get reused

[Tayrtahn]

The entity spawn panel uses EntityPrototypeViews to generate more accurate icons for the list. These entities are clientside, but they are not set as paused, so they are included in system Update loops. They also don't get reused when a new EntityPrototypeView calls for the same prototype, so more preview entities are continuously spawned as the menu is closed, reopened, and scrolled.

Notably, just pausing the entities after spawning (5bb4fa8) isn't a good fix, as this causes some of the visuals (such as solution fill levels) not to be fully set up.