Catch SIGUSR2 for reloading for settings

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Implement reloading of configurations

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Add reload menu item to redshift-gtk

Signed-off-by: Mattias Andrée <maandree@operamail.com>