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.
This button shows the menu, so it includes the folders and the link to the account options. Would a message-list-menu with "Folders and account options" value be more descriptive than "Folders" on its own?