Mac FilterShowcase refactor

These changes are meant to clean up the Mac showcase for readability and standard Cocoa/Objective-C design. The main window controller class is simplified.

Options related to view configuration are moved into new categories defined on UIImageOutput and subclasses, leveraging polymorphism. This permits removal of the pre-defined enumerated filter type, related monstrous switch statements, and pervasive class type-casting.

Filter classes are discovered automatically by scanning the classes using runtime functions.