A way to access widgets by ID / mainly canvas

We should have a way to access widgets by ID. Mainly a canvas to paint some part of its surface. E.g.

```lua
local d = Dialog():canvas{ id=myCanvas }
local ctx = d.widgets.myCanvas.context
-- ... paint in the ctx ...
```

At the moment the only way to paint in the canvas is using the `onpaint` event and triggering that event through `Dialog:paint()` (but the `onpaint` should be a way to redraw the entire canvas in case of resize, we cached the surface on a offscreen bitmap).