Review excessive repaints for threshold slider changes

Sliders are currently used in Jalview for:
- setting feature transparency (slider only) in FeatureSettings
- colour chooser (Red/Green/Blue etc, with input box alternative)
- Remove Redundancy
- threshold for Colour by Annotation, Show/Hide Columns by Annotation
- graduated feature colour threshold
- colour by conservation threshold
- colour by PID threshold
- foreground text colour threshold (slider only)

Desired behaviour where display-affecting values can be changed with a slider:
1) repaint alignment as the slider is dragged
2) update value input box (if there is one) as the slider is dragged (but no extra repaint!)
3) don't repaint structure or Overview (broadcast update) until mouse is released
4) click to move slider - update text box, repaint alignment and broadcast update (only once!)
5) update text box value (followed by Enter) - move slider, repaint alignment and broadcast update (only once)
6) update text box followed by lose focus e.g. tab out - as (5) [but mostly not done]

This issue is to review and check
(a) the right things are happening
(b) excessive repaints are not happening