Fix XACT IsPlaying properties.

Currently, XactClip and XactSound assume that `IsPlaying` is only true when SoundState == Playing. However, the wave is technically playing when it is either Playing or Paused, since pausing is still in the middle of playback (as opposied to stopping, which resets the playback position).

So, the expected behavior should be:

`SoundState.Stopped`: `Playing == false`, `IsPaused == false`
`SoundState.Playing`: `Playing == true`, `IsPaused == false`
`SoundState.Paused`: `Playing == true`, `IsPaused == true`

And there should never be a case where `Playing == false` and `IsPaused == true`.