The fit button now zooms to just one entity, and when many
entities are added at once, the camera fits to all of them.

The site wasn't checking if the entity started in bounds until after
one instance of movement.

This way, if you slightly adjust where an object is, you won't need to hold
alt the whole time. This also makes pressing alt reset this behavior, so if you
hold alt, drag out of bounds, release alt, and then move, you won't be thrown back
in-bounds.

Everything should be passing a list of entities to a function that
computes a bounding box and fits things now.

The viewer would try to call updateEntityElement, but the
entity would be deleted, causing an error.