ENH: Support 1 to 3 inputs in Register step

In order to reduce code redundancy, the register step was separated in
a widget and a step itself. The widget is not much different than the
old step as it's in charge of running the CLI.
The step is now in charge of instantiating the necessary widgets and make
sure to validate them correctly.
ENH: Make registration type available in intermediate workflow
ENH: Improve icons
ENH: Update Resample Step to handle 1 to 3 inputs
ENH: Collapse the data probe automatically
ENH: Resample step before Register step

It is better to resample the volume 2 and 3 during the registration. To
this end, we switch the resample step and the register step. The resampling
is consequently only done on the first volume.
ENH: Improve segmentation Step to allow an easier editing interface