More powerful (and simpler) image effect definition.

 * Store image effect definitions in NSUserDefaults, allowing user overrides and easier definition.
 * Image effects define an array of filters, allowing filter composition.
 * Add a "desaturate" image effect.
 * Use a new defaults key (selectedImageEffectId) since the old key was an integer.
 * Use an affine clamp before applying filters (so that gaussian blur and gloom type filters don't look awful).