Draw a circle with an icon instead of a button

Improve lock screen by drawing a translucent black circle with an
unlock icon, replacing the button.

Also add much space in the middle of the screen.
Update Weston plugin
Rename Shutdown to Power Off

Users would understand it better.
Accept both yes and challenge results from systemd

Positive results include challenge which means the user needs to be
authenticated to actually perform the operation.
Logout before power off or restart

This will close both the compositor and the shell making systemd's
life easier.
Add accepted signal to dialogs

The accepted signal is emitted when the dialog is closed successfully.
Make authentication dialog taller and make password entry bigger

Aestetics improvements.

Issue: #2
Delete session later

This will prevent the shell from crashing.

Issue: #2
Cancel authentication session when the dialog is rejected

Canceling the session is much more cleaner that setting an empty
response.

Issue: #2
Always delete objects later

Using deleteLater() will cause the event loop to delete the object
after all pending events have been delivered to it.

Issue: #2
Show user identity during authentication

We only support one identity at a time. From C++ we need something
clever to find out the best identity, meanwhile we prepare the ground
here.
Retry in case of authentication error

If the user types the wrong password we reiterate the session and
ask again.

Issue: #2
Lay out password prompt

Label and text field in a row layout.

Issue: #2
Cheap localization trick

If request is a standard label (such as "Password:" or "Password: ") we
force our own label marked for translation.

Issue: #2
Respect password echo

Do whatever the request says we should do.

Issue: #2
Show information and error messages during authentication

When PolicyKit sends an information or error message we set its
property and this will automatically make the label visible.

Issue: #2