Add support for operation log messages

Add support for operation log messages

This patch adding support for operation log messages and updating upstart
bridge code to use it. This is usefull for verbose log output (e.g daemon
starting, stopping).

Example:

 Register operation ssh:
 $ plymouth register-operation --operation-id=ssh --name='Starting OpenSSH server ssh'

 Set state of operation ssh to wait:
 $ plymouth update --operation-id=ssh --status="wait"
 Details plugin will show: 'Starting OpenSSH server ssh... [wait]'

 Set state of operation ssh to done:
 $ plymouth update --operation-id=ssh --status="done"
 Details plugin will show: 'Starting OpenSSH server ssh... [done]'

 Unregister operation ssh:
 $ plymouth unregister-operation --operation-id=ssh

https://bugs.freedesktop.org/show_bug.cgi?id=77389