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
SQUASH WITH RESAMPLE
SQUASH WITH SEGMENTATION
ENH: Create an adaptive layout

Create a different layout when dealing with 1, 2 or 3 inputs. This layout
is update through the use of the setViews method.
The workflow keeps track of all the volume displayed to make sure that
added volumes aren't shown after a setView update.
It also take care of enabling/showing the opacity slider whenever a slice
has more than one image displayed.

WIP