This is the first step in separation, so we can focus more on the
actions in this menu, and it's not tangled up in how the context menu is
shown and displayed.
Move the functionality to cache the contents of a chat bar from the room directly and to a new ChatCache object. This works pretty much the same with a few extra check and balances, this also made it easy to put a test suite around the functionality so I did. The current functionality should be identical to what exists.
This is in prep for threads which will require managing even more caches if we create one per thread.
