Logic scene list manager

It "manages" sets of choreo entities by deleting old ones.

Valve used this entity very scarcely in Half-Life 2: Episode One and once in Episode Two (but never in ) for two reasons:
 * 1) Getting rid of single-use entities which were doing nothing but taking up processing power. (This obviously wasn't necessary to do because the effect is unnoticeable, but they did it anyway.)
 * 2) Making Alyx quit talking when she no longer has to. At times, she gives the player multiple hints or prompts to do something, then stops giving hints once the player is done with the task.