Add an installer ready sound to Calamares.

The only way to add an "installer ready" sound in Calamares as seen in Ubiquity is to add it in the code as experiencing it trying to use a module has not passed for me. It can only be triggered from Calamares.

To the welcome module-a field for the path of the sound file needs to be added and then can be triggered when calamares is done and is ready-or can be configured when using settings.conf

This way the feature can be universal.