+ * Performs actions when a residue button is clicked. This manages the button
+ * selection set (highlighted by brighter foreground text).
+ * <p>
+ * On select button(s) with Ctrl/click or Shift/click: set button foreground
+ * text to brighter than background.
+ * <p>
+ * On unselect button(s) with Ctrl/click on selected, or click to release
+ * current selection: reset foreground text to darker than background.
+ * <p>
+ * Simple click: clear selection (resetting foreground to darker); set clicked
+ * button foreground to brighter
+ * <p>
+ * Finally, synchronize the colour chooser to the colour of the first button
+ * in the selected set.