Add playback progress bar

[marc1701]

Task

Add to the GUI, below the display of the selected audio clip filename, a progress bar indicating the position in the clip playback. This will be made slightly more complex by the fact that users will expect this to be interactive, so the 'playhead' position should be able to be set by user interaction.

Test

Make sure bar position indicates correct position in file, and that output from user interaction from the bar sets the correct playback position.