ScreenShield: animate new notifications

Showing the new message at full size marks an abrubt change and looks
bad. Instead, gradually animate from 0px to full natural height.
Includes hacks to workaround flickering scrollbars while the animation
is in progress.

https://bugzilla.gnome.org/show_bug.cgi?id=687660
Gdm: show error messages from fingerprint

If the user did not enroll his fingerprint, pam_fprintd will report
authinfo_unavail, which will result in a service-unavailable signal.
We can use that as a hint to control the '(or swipe finger)' hint,
and instead let error messages through, to give feedback in case
authentication fails.

https://bugzilla.gnome.org/show_bug.cgi?id=691300
UnlockDialog: show a numeric keypad when using PIN authentication

When using pam_pin.so to login with a numeric PIN, show a numeric
keypad to make it easier to type, especially on touch devices.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Login/UnlockDialog: allow choosing explicitly the authentication method

The design calls for an explicit choice between password and PIN, so
we should start both conversation and then complete the one chosen by
the user.
If PIN is not available, the chooser is automatically hidden and we go back
to password as usual.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
UnlockDialog: Improve the look of the pin pad

Use bigger, square buttons, with larger text.

https://bugzilla.gnome.org/show_bug.cgi?id=619955