media-keys: Allow the power key in more places

We allow using the power key pretty much everywhere now, but
we also ensure that we never show interactive dialogues on the
lock screen (something that was possible with other suspend/power
buttons already if more than more than one user was logged in,
or an interactive shutdown was configured).

See https://bugzilla.redhat.com/show_bug.cgi?id=980692

https://bugzilla.gnome.org/show_bug.cgi?id=707095