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
media-keys: Fix 2nd MPRIS client never being used

1. Start MPRIS client, press key, works
2. Close MPRIS client, press key, (/) pops up
3. Start same MPRIS client as 1., press key, (/) pops up

That's because we never unset "connecting" to FALSE once we were
already connected to the MPRIS D-Bus proxy.
media-keys: Make "Pause" work for MPRIS clients

There's no XF86AudioPlayPause key, just XF86AudioPlay,
even if the kernel does differentiate them (KEY_PLAYPAUSE vs.
KEY_PLAY).

We should send "PlayPause" instead of "Play" to the MPRIS client.

https://bugzilla.gnome.org/show_bug.cgi?id=705348