[Qt-creator] Drawing on QWidget

[Alex Williams]

> Create a subclass of QWidget in code and use the feature called
> "Promote to custom widget" to set your own subclass instead of a plain
> QWidget.

Thank you for help! It works, it's exactly what I wanted :)
But this process remains highly unautomated and there is no
documentation about doing this...

> You are free to use whatever toolkit you please.

I think it's a good practice to compare own product with
analog/competitors and getting best ideas from others... :)