Add MS paint example

[hatoo]

Since #140 has landed,
I realized that we can make MS paint like example by using lyon now.

This PR resolves #18

[artursapek]

hah, awesome

[hecrj]

Thank you for giving this a shot! It's great to see the new primitive can be used to create stuff like this very easily.

I left some feedback that I think we should address before merging. Let me know what you think!

[hecrj]

The widget generates strokes every time the cursor is moved while dragging. It emulates a pencil. This means that strokes will add up very quickly and will be very close to each other.

I am not sure implementing a pencil like this is the best way to showcase how to combine the new Mesh2D primitive with lyon. My intuition tells me that something like a color buffer would work better for this particular case.

I believe shape tools are the ones where lyon can shine. Maybe we could implement line/rectangle/circle tools instead?

[hatoo]

I agree with that "pencil" is not a very good example which demonstrates the combination of iced and lyon.
I choose to make a bezier tool as an example to demonstrate it instead.
Because I think a tool which is supporting only bezier curve is enough to show power of lyon.

[hecrj]

Cool! A bezier curve tool is a better use case for lyon indeed!

I have improved a bit the state transitions in the Bezier widget and made it display some instructions when completely empty.

Let me know what you think and we can merge this!

[hatoo]

Thanks! Codes became more decent.
Please go ahead!