Implement and bind buffer-move-{next,previous}

Implement and bind buffer-move-{next,previous}

Control + Shift + PgDn now moves a tab to the right.
Control + Shift + PgUp now moves a tab to the left.

These keybindings are consistent with Chromium, Firefox,
Nautilus, and gnome-terminal.

Note that I allow for wrapping. This is consistent with
allowing wrapping for buffer-{next,previous}, but there
might be reasons to disable it in the future.