Since commit 6fefe75a1c the handling
of forward-focus bindings happens before the resolve_expressions pass,
which means that we cannot rely on type mismatch
error handling yet.
Fixes#4475
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.
