Bug 1068695 - added collapsed/expanded semantics to button to bring up f...

...older picker.

In here changing message-list's links semantics to button since it makes more sense for collapsed and expanded state and for consistency with the folder-picker card.
Yeah, in this case we want to indicate that we have an expanded state and the label/name should be consistent with what we have when the state is collapsed (e.g. menu or folders).