Scan PPM partitions on double-click

[fadden]

Pascal ProFile Manager (PPM) partitions are discovered automatically. However, if the depth is set to "shallow", there's currently no way to cause them to be scanned manually. The partition "file", which has a unique ProDOS storage type, is currently marked as damaged to prevent anything from messing with it. This is good, but we should treat a double-click on it as a desire to scan the embedded volumes.