factor out PHScaleColors.PANEL_FILL, color for all panels and expand/collapse bars
bug fix in BasicsPHMeterNode, 'enabled' fill was null
1/14/14 add option.emptyWhenSoluteChanges to Solution, needed for 'Custom' screen where a new custom solute is created as the pH changes
1/14/14 make Solute immutable, change pH of 'custom' solution by creating new Solute instances when the pH changes