Issue 1315

Id's are removed as this components are now being used more than once.
The on enter key action is replaced with a traditional button that becomes active when the user enters both values. The error labels are now redundant.
This is really really close, I promise :-)  

But... I think the event name should reflect the action that happened, i.e. that the colour scale was updated (`colourScaleUpdated`?).  What any other components do when they get the event is their business (in this case, the `StylePanel` refreshes its legend).