Iop exposure - deflicker update

Deflicker mode explanation:
* DEFLICKER_MODE_THUMBNAIL - this is what been implemented initially. exposure compensation is computed in GUI from histogram (same histogram as in levels iop) and stored as numeric value
* DEFLICKER_MODE_SOURCEFILE - exposure compensation is computed in GUI from histogram of original RAW source file and stored as numeric value.
* DEFLICKER_MODE_AUTOMATIC - on export, in process() of exposure iop, calculate histogram of ivoid and calculate exposure compensation based on that histogram. **WARNING: using this mode, DT can now automagically calculate proper exposure compensation for each processed image without any user interaction - extremely useful for timelapses and video frames**