hdf5: merge images with identical properties into different channels of same image

Should be easier to open and manipulate with Huygens
gui.util: move the img operators to there

every img operator should go there. Still need to write them... maybe using VIPS?
gui: generalise image drawing on canvas for >2 images

Need to differentiate between SEM and Fluo images => SEM image is always the first one (and there should be only one)

Seems to works pretty fine for quite a hack. However, its performance can be optimized for sure. At least, we should
cache the sum of all the fluo images if they haven't changed.