[PATCH] emacs: notmuch-hello: Use our own keymap.

[sebastian at sspaeth.de]

> Previously, we were adding our bindings directly to `widget-keymap',
> resulting in them being shared by anything else that used that keymap.

Is this patch preferable over the one I sent some time ago in
id:1272882573-9864-1-git-send-email-Sebastian at SSpaeth.de ?

My patch makes notmuch-hello a major mode and also creates the proper
keymap. THe major mode allows the '?' keybinding to also work.