emacs: tree: make b bounce a message and backspace scroll message pane up

This tweaks the keybindings in tree-mode. It make b do bounce/resend
matching show-mode. Since b was already bound to scroll message pane
back, we now use backspace for that.

This means space/backspace scroll the message pane forwards/backwards,
and page-up/page-down scrolls the tree pane forwards/backwards.