Some improvements to ConfigurareTrackCameras dialog box and process.

Fixes #37

* Begin capture button does not become enabled, if the track cameras have
  not been defined for the currently loaded track.

* An error label is shown if the track cameras have not been defined for
  the currently loaded track

* Fixed a bug in the Track Configure Dialog box.  If the track's cameras
  had changed, the new cameras would not be showing in the dialog box.

* Shows an error message if user has not configured more than 25% of
  cameras.