BaseWidget should ensure BackgroundColorListener is set so that Grid will be updated

[rjackey]

Add this to constructor to ensure background color listener is still set if the concrete class doesn't modify the BackgroundColorableComponents. This ensures it will still update the color of the Grid on background change.

        function obj = BaseWidget(varargin)

            % Attach internal postSetup callback
            args = horzcat(varargin, {"CreateFcn",  @(src,evt)postSetup_I(src)});

            % Call superclass constructor
            obj = obj@matlab.ui.componentcontainer.ComponentContainer(args{:});

            % Ensure background color listener has been generated
            if isempty(obj.BackgroundColorListener)
                obj.updateBackgroundColorableComponents()
                obj.listenForBackgroundChange();
            end

        end %function