Asynchronous messaging for mouseovers and linked brushing

A number of operations in Jalview can trigger updates in other linked windows that cause the UI to block. These include sending of selections and mouseovers, and most obviously, colouring and visibility updates which are propagated to linked structure views.

Ideally, all messages should be asynchronous - allowing the user to continue interacting with the current view without blocking, and also allowing windows to implement a leaky pipe to maximise responsiveness (e.g. only the last received mouseover needs to be shown/scrolled to in a linked window, or only colourchanges that *actually change the structure view* need to be sent on to the structure viewer...).