In combination with running a single instance covered in PICARD-2471 consider allowing starting processing actions from the command line.  Actions could include:

• SCAN - Scan all files in the cluster pane.
• CLUSTER - Cluster all files in the cluster pane.
• LOOKUP - Lookup all clusters in the cluster pane.
• SAVECOMPLETE - Save all completely matched releases in the album pane.
• REMOVESAVED - Remove all saved releases from the album pane.
• FINGERPRINT - Calculate acoustic fingerprints for all (matched) files in the album pane.
• SUBMITFINGERPRINTS - Submit outstanding acoustic fingerprints for all (matched) files in the album pane.
• SHOW - Make the running instance the currently active program.
• QUIT - Exit the running instance of Picard.