settings: Make Enter work in pin confirmation dialogue
lib: Override default icon for audio devices

"audio-card" is fugly, low-res and represents something that
nobody's seen for 10 years: a stand-alone sound card.

Instead, we'll show headset, headphones, and speakers to represent
generic audio devices.
settings: Defer pairing when the name isn't known yet
settings: Avoid warnings when discovering new devices

When device names aren't known yet, don't throw warnings.
settings: Never have nameless rows

When the name of the device isn't known yet, don't show a NULL
row, show the device type instead.
settings: Unregister the pairing agent on exit