Don't hint messages in nnir or virtual summaries

We probably got there from `gnorb-org-view', we know they're relevant.

* lisp/gnorb-gnus.el (gnorb-gnus-hint-relevant-message): Don't hint in
  nnir or nnvirtual summary buffers.
New function for restoring window layout

* lisp/gnorb-utils.el (gnorb-restore-layout): Interactive function for
  restoring window layout.

* lisp/gnorb-gnus.el (gnorb-gnus-incoming-do-todo): Delete other
  windows, so we're left with a clear frame. `gnorb-restore-layout' can
  bring us back.

* README.org: Document.