Ideas for improving canvases that live in the browser

[almarklein]

Affects the pyodide, anywidget and native browser backends.

We already have:

• set_css_width
• set_css_height
• set_resizable
• show_titlebar

Ideas:

• Minimizing the canvas, so it doesn't render temporarily.
• Maximizing the canvas, to use the full viewport.
• Manually closing the canvas.
• Show a hint when the canvas has focus. E.g. make the titlebar slightly more pronounced.
• Pyodide-specific: spawning new canvas in a container element.
• Also see https://github.com/pygfx/renderview/issues