Rename vessel extraction to vessel enhancement

Also revisit this placeholder and update it further now that we know that
it will be used with EnhanceUsingNJetDiscriminantAnalysis.
Add ExtractSkeleton step:

Just a placeholder for now
Add Vessel Extraction Step

Just a placeholder for now.
Refactor settings panel

Set the reload button visible in advanced workflow only.
Also, the volume currently viewed by the user are a little bit more
enforced b re-setting the last viewed volume after each step and everytime
the module is shown.