Renamed test case module for image viewer for consistency.
Extracted common functionality of image viewer's left click states to 
a superclass.
Renamed the base state in the image viewer to BaseInteractionState.
Refactored AdjustColormapState to use this base class as well.
User can now left click with dragging to center the image viewer circle
on their click.

Implemented by adding image viewer state to handle recentering, and
having the other left-click states extend it.
Merge pull request #37 from drusk/centre_circle_on_click

Centre circle on click