Add feature to append Scheduler jobs to queue.

This MR adds a push button to the Scheduler toolbar, near the load/save buttons.

This button simply appends the contents of an ESL file to the queue, instead of removing all jobs prior to adding the new ones.

The new button icon is document-import, closest intent I found. The other existing buttons do not change their behaviour. The D-Bus interface does not change neither, although there is no way to append jobs from that side.