The list of displayChange listeners in WDisplayChanger is currently set
up as an ArrayList. This works fine for most situations; we add items,
we remove items, we iterate through the list sending displayChanged
events. But in a multi-threaded situation, this could break down
because we may try to do several things to the list at once.

For example, we might, during a displayChange, try to add items to
the list at the same time as we are iterating through the list to send
the message. This causes a "comodification" error on the ArrayList
object and throws an exception, causing rendering and execution problems.