Introduced image state to validation model which will allow the model to
be switched between a singlet and triplet state.
Added triplet state to model, and methods for switching states, which
are called by the controller.
Image manager keeps track of workunits already submitted for download.
Switching between states causes downloads of the old state to be halted
and downloads of the new state to be resumed.
Added ViewerManager to MainFrame for handling creating, showing 
and hiding the two potential image viewers.
Renamed focal_point to focus.
Renamed cutouts.triplets module to cutouts.grid.
Fixed ImageCutoutDownloader convenience import, and made 
planted_mag_check.py use it.
Fix keyword in test.
Disabled Image triplets menu item under Display menu.