Also switches from setting an invalid attribute to using
the attribute dataset to store the old unit for the unit
selectors and defers loading a scene until everything
is configured.

Firefox won't draw SVGs to a canvas correctly without width/height
attributes (even if a viewBox is present). This commit inserts those
values where needed and removes the Firefox check from the click
detection code, so it works properly (and screenshots are fixed, too).

Entities positioned above the ground were going off the screen.

There are probably still problems in the x-direection.

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.