Implement dialogs to edit valueColors for 2 multiColorDots plot types

#59 (closed)

Implement MultiColorDotDialog which is a subclass of EditValueColorDialog to be super class for the 2 following class:

• MultiColorDotLowerEqualDialog is implemented to edit value for multiColorDotsEqualOnLowerBound plot type.
• MultiColorDotUpperEqualDialog is implemented to edit value for multiColorDotsEqualOnUpperBound plot type.