Ask for authentication only once

Fix bug 428974

With this MR, polkit authorization is asked once before any command that needs it and, if not given, skips the device scanning procedure.

This fix also an additional bug in which, if you canceled the huge number of authentication pop-ups that appeared before and then manually scanned for devices, partition manager warned you about the missing of the tools to handle your file systems (because it didn't reinitialize the FileSystemFactory using the new permissions).

In addition, now, the "Select Current Device" menu is disabled if no devices are available.