Custom background color of widget should be set upon reloading app in AppDesigner

[slootsjj]

Recreate issue
Create app in AppDesigner
Add widget to app
Change background color to blue
Save and close app.
Load app again in AppDesigner.
--> Background of widget appears as default color.

Proposed fix
Listen to 'PostUpdate' event in BackgroundColorable mixin class to ensure a widget's custom background color appears in AppDesinger upon reloading the app. Changing the background color of the widget will trigger the widget's update method as well.

function listenForBackgroundChange(obj)

    % Establish Listener for Background Color Change
    if isempty(obj.BackgroundColorListener)
        obj.BackgroundColorListener = ...
            addlistener(obj,'PostUpdate',...
            @(h,e)obj.updateBackgroundColorableComponents());
    end

end %function