Relay SIGUSR1 to redshift from redshift-gtk

Current, if redshift-gtk receives SIGUSR1, redshift-gtk will terminate.
With this patch if redshift-gtk receives SIGUSR1 it will send it to redshift.

This patch should close:
https://bugs.launchpad.net/redshift/+bug/663637 (well I do not know what
a systemshortcut is but it sounds like something that should be done with
xbindkeys.)