emacs: show: make buttons select window

Emacs has two button type objects: widgets (as used for saved searches
in notmuch-hello) and buttons as used by parts/citations and id links
in notmuch-show. These two behave subtly differently when clicked with
the mouse: widgets select the window clicked before running the
action, buttons do not.

This patch makes all of these behave the same: clicking always selects
the clicked window. It does this by defining a notmuch-button-type
supertype that the other notmuch buttons can inherit from. This
supertype binds the mouse-action to select the window and then
activate the button.