factor out canvas backing logic for widgets

[suno lili So]

create a new mixin called e.g. CanvasBacked that performs the canvas drawing logic for a widget. unexpose Widget.get_canvas, and instead expose:

• a CanvasBacked.refresh method, which will do effectively what Widget.draw now does; and
• a Widget.draw method, which will draw a widget at specified coordinates on the screen or active canvas

note that there will be no CanvasBacked.get_canvas. this is because with CanvasBacked's implementation of draw, there will be no need to leak the canvas. for that matter the mixin can be called something more centered on its extrinsic properties instead, such as Buffered

the idea here is that widgets can be designed to be buffered or not depending on their needs. like it's probably not necessary for a simple button widget to have its own canvas

[suno lili So]

resoloved by 7aabbd6