Add IMP_Imap_Tree_Element::.

Add IMP_Imap_Tree_Element::.

Since we don't need all information every time we access mailbox element
information, this adds substantial overhead.  Instead, convert the
return from IMP_Imap_Tree#element() to a IMP_Imap_Tree_Element object
which does most of the necessary overhead only when the data is actually
accessed.

Clean up some things while doing this.  Namely, IMP_Imap_Tree#build()
is essentially duplicative of the more flexible
IMP_Imap_Tree#listFolders() so ax the former.