Widget improvements after meeting from 18th April

These are the possible improvements gathered from the beamline feedback on 18th April:

• Extract SPEC/HDF5 path definition in a separate widget !8 (merged)
• Extract axes definition in a separate widget !10 (merged)
• Extract axes choices in a separate widget !10 (merged)
• Adapt the visualization of the Collect tasks to the data dimensions (multiple axes mean multiple dimensions) Tracked at #5