automountManager: Explicitly track active operations

As a mount operation's UI may be reused (for example after mistyping the password), we only close the operation once the mount has finished (successfully or with error).

We therefore need to track ongoing operations, which we currently do by monkey-patching the corresponding volume object. However while the underlying GVolume object indeed remains the same through-out the operation, the JS wrapper object isn't referenced anywhere and may thus be garbage collected, resulting in a stuck dialog.

Fix this issue by tracking active operations explicitly, so that all involved objects are referenced until the end of the operation.

Fixes #565 (closed)