diff icons/speaker.png @ 164:75cafe7a9246

* Make it possible to "measure" a feature on the spectrogram by double-
  clicking in measure mode
* Make shift-click-drag (for zoom to region) work in measure mode as well
  as navigate mode.  It would be nice to be able to shift-doubleclick to
  zoom on a feature directly using a combination of these last two features,
  but that isn't possible yet.
* Make Del delete the measurement under the mouse pointer.