workflow

When user will select the kv file to open, 
then all the root widgets(as they will be the custom widgets) will come in the Toolbox.
These root widgets will also be displayed by the playground. 
Each root widget will have its own playground where user can edit it. 
To display these root widgets what we can do is:

    1. Have a Tab Widget which will contain each playground for each root widget.

    2. Have a Tab Widget containing individual Toolbox, Playground, 
    WidgetTree, StatusBar and PropertyViewer for each root widget

    3. Create a BoxLayout and place ToggleButton in it which will display the
    name of root widget and clicking on any of them, then Playground for
    that root widget will be displayed (I think this one is better)
    
There will be two ActionButtons "Create New Root Widget" or 
"Open New Root Widget" to add a new root widget to the current kv file.

Saving the file will create kv file and will update py files also.