Initialize ctimes list in gcylc dot view.

Bug fix: attempt to *View -> Transpose* on an empty dot view (prior to starting the suite) results in
```
Traceback (most recent call last):
  File "/home/oliverh/cylc/cylc.git/lib/cylc/gui/DotUpdater.py", line 371, in update_gui
    self.ledview_widgets()
  File "/home/oliverh/cylc/cylc.git/lib/cylc/gui/DotUpdater.py", line 226, in ledview_widgets
    types = [str] + [gtk.gdk.Pixbuf] * len( self.ctimes )
AttributeError: 'DotUpdater' object has no attribute 'ctimes'
```