shell: Use its own timer for the screensaver timeout

shell: Use its own timer for the screensaver timeout

Previously, the shell would change the idle_time of the compositor to
the screensaver timeout and wake the compositor again, so that after
the timeout the compositor would fade to black and then the shell would
receive the lock signal again and would set the dpms state of the
outputs to off.

Instead, use a screensaver-specific timer for that, and call lock
without the back and forth between the shell and core Weston.