ScreenShield: switch resetLockScreen to accept keyword arguments

Having two booleans as argument is just confusing.

https://bugzilla.gnome.org/show_bug.cgi?id=699112
Lightbox: have animation times passed as parameters to show() and hide()

This way it is possible to use the same lightbox with different
times.

https://bugzilla.gnome.org/show_bug.cgi?id=699112
ScreenShield: fade the screen to black when locking manually

When locking manually (or locking with an animation), fade the
screen to black after a small timeout. This provides a smoother
experience, instead of abruptly turning off the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=699112