Register Step:

The Register Step uses the two given images and co-registers them.
Since we are using BRAINS for the registration, we need to build it with
Slicer.
The Register Step was thus fitted to use the BrainsFit CLI. So far the
options allowed are limited to the inputs as the module used might be
changed in the future.

This is also the occasion to add some infrastructure to the WorkflowStep
and the Workflow. A progress bar is added above the advanced panel.
Segmentation Step:

This step is right now fairly empty. It will be filled later with the
Editor modules
Vessel Extraction Step:

-- WIP --
Fix comboboxes sizing.

The comboboxes on the same row as save button or open folders are enlarged
beacause of the icon size. It causes a displeasing discrepancy in the
display.
Add coice of registration type

Add a combobox for the user to choose the registation type in advanced mode