sound: Fix port handling for the unknown availability case

The current code assumes that port availability transitions will be
between YES and NO, and doesn't account for the fact that it may also be
UNKNOWN. This causes spurious entries if the port availability
transitions between YES and UNKNOWN.
Fix icons on Sound preferences dialog
Fix faces location on User accounts dialog
Use freedesktop as default sound theme instead of ubuntu
Merge pull request #4 from City-busz/various-fixes

Various minor fixes