Fix launching of debug mode; need to create inspection tool before 
starting the wx app main loop.
Image view manager is now part of the ApplicationView instead of 
MainFrame.
ImageViewManager now creates the matplotlib figure canvas and its is
shared by the singlet and triplet viewers.
Added method for submitting triplet download requests on retries.
Image loading dialog is displayed when switching between singlet and 
triplet mode if the image isn't already loaded.