cylc gui: dot view redraw resetting selection

[arjclark]

Somewhat frustratingly for users with large suites using the dot view of cylc gui, it seems that whenever a redraw happens the view gets reset so you are no longer focused on the area of interest.

@benfitzpatrick - I seem to remember we had a similar problem with treeview, can you advise?

[benfitzpatrick]

The tree view is nearly always incrementally updated rather than cleared and rebuilt - I think the dot view needs the same approach.