gvc: Add "what did you plug in" support API

Add "audio-device-selection-needed" which will be emitted when a
headphones, headset or microphone is plugged into a jack socket that
cannot detect which type it was.

Once the user of libgnome-volume-control has asked the user which type
of device this was, they can call gvc_mixer_control_set_headset_port()
to switch the ports for that configuration.

Note that gvc_mixer_control_set_headset_port() supports passing the
card ID, but the detection code only supports a single such device. When
we find hardware that can support > 1 such device, we can test and
implement support without breaking the API.

Based on the original code by David Henningsson <david.henningsson@canonical.com>
for the unity-settings-daemon

https://bugzilla.gnome.org/show_bug.cgi?id=755062