ENH: Control default settings in vesselview

With this change, we can control the default user settings in the
application without having to modify slicer's code. It works by checking if
a setting file already exists or not. If one already does not exists
already (for example the first time vesselview is started) then the
VesselViewDefaultSettings.ini is used. However if one exists already it's
left intact. It's thus transparent for the user as they can style change
the settings however they want.
FIX: Fix call to observer in ResampleWidget on cancel
ENH: Add Save result text to the save buttons

As a result of this, the tool button were promoted to push buttons.
Note that the "Save results " text as an extra space in the end to provide
a more balanced-looking widget.
ENH: Bigger, greener text for the apply push buttons

In order to make the action more visible/obvious to the user, the push
button were costumized with:
 - a green-colored gradient inspired by the next/before arrows of the
workflow
 - Two spaces before and after the action to enlare the button and
highlight the text
 - an bigger point size (as the application default point size is 12)
FIX: Fix VesselEnhancement step getFileNameFromVolume function
ENH: Finish/Update the VesselExtration step

For this, add another layout that has a 2D view and a 3D view.