Fix category visibility

[mxgrey]

Right now when category visibility gets toggled, we have a naive system that directly toggles visibility of all elements of that category in the site. This can interfere with elements whose visibility is contextual, e.g. lanes that are on a different level should still not be visible when lane category visibility is turned on, but currently toggling lane visibility off and then on will force all lanes across all levels to become visible.

Instead the individual systems that update visibility should look at their CategoryVisibility<T> resource and factor it into their visibility update decisions.

[mxgrey]

As a temporary workaround for a related glitch, #415 removed the system that updates visibility of new objects based on their category visibility. This was necessary because otherwise race conditions could cause lanes to become visible on levels that aren't the current level.