Track Properties Dialog no longer blocks interaction of the main window.

Track Properties Dialog no longer blocks interaction of the main window.

- Before this change, bringing up a track dialog would stop users from
  doing anything else with viking. The restriction is mow removed.
- The dialog is no longer in modal mode.
- Multiple dialogs for different tracks can be up at the same time.
- Users can only start one dialog for each track. Once its dialog is on,
  its  "Properties" menu item will be disabled.
- When a track is deleted, its dialog (if on) will also be destroyed.

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>