Bugs 994, 1030: Scrubbing behavior with pointer outside of the track panel...

... first: suspend scrubbing only when the application is deactivated -- not
necessarily when track panel loses focus; resume when reactivated.

second: click and drag to seek happens only when the pointer is in the track
panel, and either over a wave track, or over no track.

src/Envelope.cpp

@@ -1064,7 +1064,7 @@ void Envelope::GetValues(double *buffer, int bufferLen,
    t0 -= mOffset;
 
    // JC: If bufferLen ==0 we have probably just allocated a zero sized buffer.
-   wxASSERT( bufferLen > 0 );
+   // wxASSERT( bufferLen > 0 );
 
    int len = mEnv.Count();