Helper for checking every member of a collection against a predicate

[Robdel12]

It would be nice to have a way to check every member of a collection against a predicate that returns the results &&ed together.

Given a collection such:

  items = collection($selector, {
    isSelected: hasClass('selected', $child)
  });

The ability to add something like the following property would be boss:

  allItemsSelected = all(this.items(), item => item.isSelected);

(moved from thefrontside/bigtest#65)

[wwilsman]

I don't think a helper for this is necessary, and it would get complicated in relation to this.

We could simply use a getter instead :)

items = collection($selector, {
   isSelected: hasClass('selected', $child)
});

get allItemsSelected() {
   return this.items().every(item => item.isSelected);
}