shortcuts: Enable Ctrl+PgUp/PgDn when multiple tabs

Users expect Ctrl+PgUp/PgDn to switch tabs, but some terminal
applications want to be able to use these keys themselves.  Make the
same compromise that gnome-terminal does, and let these keys switch
tabs _if_ there are multiple tabs in the current window to switch
between.

Fixes #118.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
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