Wire up toggle-all logic

There's a bug here I haven't yet found the source of: the first time you interact with the toggle-all checkbox, it's no longer alterable via the everything-completed? message.

That is to say...

Toggling individual TODOs as complete has the desired effect of causing the toggle-all element to reflect their aggregate completedness.

The toggle-all element itself has the desired effect of controlling TODO completedness en masse.

However, interacting manually with the toggle-all element has the undesired effect of disabling the first behavior.

It's as though domina/xpath refuses to change the :checked attr for a checkbox the user has altered manually.