internal: an alternative to default_focus

internal: an alternative to default_focus

Interfaces can now set event listeners on their window for 'tab_focus' instead
of defining a single default_focus function.

We also include a change to browser.js so that embedded interfaces also get
their default_focus function called (and a tab_focus event sent).

This is geared to help future functionality, but shouldn't produce any end-user
visible changes by itself.

Signed-off-by: Jason Etheridge <jason@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>