Changed value of MapVM.ColoredCells map from Brush to CellColor

Type of ColoredCells changed to IObservableDictionary<GridCoordinate, CellColor>.
Added CellColor? property to CellVM

Encapsulate a cell view model's "color" in new property CellColor.
Right now, still wraps a call to MapVM.GetCellColor, but this will
be refactored into a real property on CellVM (with property-change
notifications.)
Added MapVM.AddCellVM method

New MapVM method AddCellVM allows adding a single cell view model.
Removed MapVM.ColoredCells collection

Cell color is now stored as a proper property on the CellVM view model.