CanvasView: Make the canvas the main element

[martinRenou]

This removes the extra div container, and it means users can now apply the Layout property on the Canvas itself instead of the extra container.

This means that when the user does:

canvas.layout.width = '100%'  # Take all the available width
canvas.layout.height = 'auto'  # Automatically find a suitable height (respecting the ratio)

the canvas will have a "responsive" behavior that respects the available space AND the canvas size ratio.

When doing:

canvas.layout.width = 'auto' 
canvas.layout.height = 'auto'

which is the default, the canvas will respect its width and height properties.

And when doing:

canvas.width = 200
canvas.height = 200
canvas.layout.width = '100px'
canvas.layout.height = '100px'

the canvas will take 100x100 pixels on the screen, but will actually contain an image of size 200x200.

This should fix #110 and Kitware/ipyvtklink#2, which should both use the first approach.

[martinRenou]

The MultiCanvas will not behave the same way. We need to find a workaround for it.

[martinRenou]

Let's merge this as it is. And see what to do for the MultiCanvas later.