Achieve this by generating a `focus()` function for such components
and call it from the outside.
This replaces the previous focus handling with what should be cleaner:
- Any `forward-focus: some-element;` is basically syntactic sugar for
`public function focus() { some-element.focus(); }`.
- The init code gets simplified to calling focus() on the root, if it's
available.
Since the `focus()` functions are now generated in the imports pass,
they become visible in the style checker. That means the checker
requires consistent focus handling between the styles.
- Remove extra padding from the todo demo. Box in Box causes the
padding to be added twice otherwise. And the Box have extra big
padding with the material style.
- Fixup the combobox size to be consistant with fluent wrt sizing
- Use the proper color for the checkbox border, following the
style specification
