add feature to allow configuration of panes

[parosen]

it would be helpful to be able to tell qed how data sets should be displayed. Example use cases:

1. multiple panes, each with a different polarimetric channels from a single image, synced, and with a common mapping of DN value to color/grayscale.
2. multiple panes, each with a functional on channels (e.g. HH/HV amplitude ratio) from images acquired at different times, synced, and with a common mapping of DN value to color/grayscale.