Harmonize UI to slicer's standards

 * Group the inputs (optionnal or not) in the collapsible button and the
result in another.
 * Move the apply button to the lower right corner like in CLIs
 * Change the save icon for the text "Export" (now a QPushButton instead of
a QToolButton)
WIP - Display only the meaningfull statistics

When running without the grayscale node, some statistics aren't meaningful
at all and crowd the interface.

BALBALBABLBALABLABN
...