Add drawing pane widget

[jrmi]

No description provided.

[Ribakys]

This feature could be very interesting.

It should also work on mobile or tablet.

For example, for a game inspired by "Esquissé" could use this feature. Thus, we could have a new widget named "Container" with different operating modes.
The "Stack" mode allows elements to be stacked, for example a succession of "drawing pane" widget and text entry widget (with text locking afterwards). And a button would go to the next item, and another button would go to the previous item. One last button to reset the content of these widgets.

[jrmi]

Some links:

• https://stackoverflow.com/questions/40595783/emulate-free-drawing-with-fabricjs
• https://plnkr.co/edit/V1c1xkB29tgB2ha99CRQ?p=preview&preview
• https://stackoverflow.com/questions/2368784/draw-on-html5-canvas-using-a-mouse
• https://jsfiddle.net/cranes/kp74m47d/
• https://stackoverflow.com/questions/6770899/javascript-library-for-free-form-drawing
• https://github.com/fabricjs/fabric.js
• https://github.com/federico-moretti/canvas-free-drawing