screenShield: Only allow graphs to lift shield when locked

Since commit 67615a0c, any printable key can be used to lift the
screen shield rather than just escape/enter/space. While this is
convenient for unlocking where the input is forwarded to the password
entry, it is potentially dangerous when the screen is inactive but
not locked: If the user types her password, the first character will
lift the shield and the remaining input will go to the focused window.
To prevent this from happening, only allow printable keys when the
screen is actually locked and restrict the key presses that lift the
shield to the original set of escape/enter/space otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=773328