Update selectedPalette in all other instances also on open

The spectrum colorpicker has already the option 'localStorageKey'. After reloading the page all instances with the same storageKey update their palettes with the chosen colors.

With my changes, the palettes gets updated also on open. That means, when there are more than one spectrum instance with the same storageKey and the user chooses a color in the first picker, he will see his selection immediately in the other picker after he has opened it.

Since the instances already get updated after a reload, this changes are a logical consequence for a multiple instance usage.

The changes are covered with two new tests.

I would be happy, I you could merge my request! I am looking forward for your feedback!