Preprocess Image widget

[borondics]

We shall make a new widget that will act on 2D or 3D data but the operations will aim at the XY image dimension.

Features should be similar to Preprocess Spectra from the orange-spectroscopy add-on.
We will have a list of preprocessors that can be added on the left control area.

The right side should display one image but have two tabs that will represent the state of data before and after the selected preprocess step or the entire workflow, if no preprocessor is selected.

The image display should be the same as the image of the Hyperspectra widget from orange-spectroscopy.

[borondics]

@markotoplak if you could make the structure and logistics of the widget we will start filling up the preprocessors.

[ngergihun]

We also realized that we don't need the "Show spectra" option (red rectangle in the attached image). Instead we all the time need all the pixels. @markotoplak could you at some point remove this? So we always use all the data in the processing.

Another question: can we access the current values of the "Axis x", "Axis y" comboboxes from the preprocessors? In order to get that specific columns from the data if needed for the processing?