Add a timeout to space-cadet shift.

Add a timeout to space-cadet shift.

When one holds a Space Cadet shift, to have it act as a shift, so that
mouse behaviour changes, when released without any other key pressed, it
still registers a paren. To remedy this, add a hold timeout: if the key
is held longer than TAPPING_TERM, it will not register the parens.

Fixes #884, with the side-effect of not being able to have parens
trigger the OS-side repeat anymore.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>