emacs: show: modify the way hidden state is recorded.

Previously, whether a part was hidden or shown was recorded in the
invisibility/visibility of the part overlay. Since we are going to
have lazily rendered parts with no overlay store the hidden/shown
state in the part button itself.

Additionally, in preparation for the invisible part handling move the
actual hiding of the hidden parts to insert-bodypart from
create-part-overlays.

Finally, we will need to know whether a part-insertion has done
anything (it won't if the invisible part cannot be displayed by emacs)
so we slightly rejig the code order in
notmuch-show-toggle-part-invisibility to make it easier for the
function to set an appropriate return value.