Provide feedback during unlocking in GtkPlacesSidebar

During the user testing of the VeraCrypt beta, 4 participants out of 5 were confused by the lack of feedback while unlocking a VeraCrypt volume.

The lack of feedback is slightly confusing when the unlocking parameters are correct because the operation is shorted and the outcome is happy.
The lack of feedback is clearly problematic when the unocking parameters are incorrect because the operation is much longer. People don’t understand what’s going on, they start doing other things, and sometimes even fail to relate the error message to the VeraCrypt operation as it comes so long afterwards.

Original created by @sajolida on 15664 (Redmine)

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