improve user feedback when unlock with fingerprint

[andirsun]

Problem

Currently the greeter is unlocking correctly with my fingerprint sensor but no exists any confirmation check when the laptop unlock

Proposal

Show a confirmation icon like emblem-default when the fingeprint sucess the check.

Prior Art

[ibnesayeed]

My experience goes like this:

• Greeter does not show any signs of fingerprint auth initially
• First attempt of password auth (with correct password) fails
• Fingerprint icon appears after that
• Fingerprint auth works