Switch to observer pattern for colouring the structure view to avoid queueing lots of updates

For complex structure views, any change in structure coloursheme can take more than 0.3s to be processed by the viewer. These take place via a Swing.invokeLater task (which contains the series of commands to be executed by to viewer), since there are coordinated updates to progress bars or other windows.

If several updates are triggered (say by dragging/releasing a threshold slider several times in quick succession), then update tasks can stack up on the Swing worker queue - slowing down the UI and resulting in the user having to wait till all the updates are complete.

It would be better if the structure view update task were coded as an observer - that requests the current state of the view and only updates the structure view if there has been a state change.