emacs: Add a notmuch-saved-searches function.

emacs: Add a notmuch-saved-searches function.

We use this function to abstract away the common 3-step process for
looking for a value for the saved-searches variable:

	1. Look at the notmuch-saved-searches variable itself
	2. Look at the notmuch-folders vaiable
	3. Use a default value

We were already using this logic (open-coded) in notmuch-hello, but
notmuch.el was accessing notmuch-folders directly for the clever name
selection of search buffers.