Switches of the new Persistent Storage are too slow to respond on some USB sticks

See these screencasts:

switches_1

switches_2

Once I think that this triggered an error message related to DBus-something.

I understand that things are happening in the background while the switch reacts to the change and that's fine.

To prevent people from wonder what's happening and turning switches on and off in response (like I did), we should communicate more clearly while the background work is happening:

• Add a spinner on the left or right of the switch
  • Adding it to the left might be easier in terms of layout, but might require moving the other icons that are on the left (folder, settings, etc.)
  • Adding it to the right, in the gray margin, might be harder in terms of layout
  • I don't think that it matters much in terms of UX, so the easiest to implement, the better
• Freeze the switch so the user cannot switch it back and forth

Priority: ideally in 5.8.