'sleep' can now appear in the data field of a 'time' shortcut
without being followed by a number, allowing you to stop a
running timer, or to start a new one using the default sleep
timer duration (the duration setting can already be added to
the Shortcuts menu as well).
Also see here:
https://forums.rockbox.org/index.php/topic,54312.msg250940.html
Change-Id: I9d0e62ef1b6187c35133067349729a4d94273c7a