Move Image Calculator to its own package

This is in preparation for additional calculator-related classes being
added, so that they can all stay grouped together.
ImageCalculator: clean up style
ImageCalculator: make operations extensible

There is now a new type of plugin, CalculatorOp, for image calculator
operations. All existing operations are now defined as CalculatorOps.
Merge branch 'image-calculator'

This branch makes Image Calculator operations into their own plugin type
(CalculatorOp), so that it can be easily extended with new operations.