Code review comment for lp://qastaging/~mitya57/indicator-sound/lp1502480

Not a blocker, but I think the standalone ``is_unity()'' function makes sense here as well as in indicator-datetime.

Two reasons:
1. readability: (is_unity() && (Environment.find_program_in_path ("unity-control-center") != null))
2. the ability of is_unity() to cache its value s.t. it doesn't have to re-evaluate each time it's called