ENH multiple things :

- deleted removeArrow() in gradientWidget
- added the bool variable m_updateOnPositionChanged in ShapePopulationQT to avoid unecessary updates when a color arrow is moved
- added the bool variable m_renderAllSelection in ShapePopulationBase to be used anytime something is rendered, to avoid calling RenderSelection() for nothing : way faster!!
- Changed loadVTKfileCLP into loadVTKfilesCLP, seo that using CLP or Drag&Drop, createWidgets is only called once all the files are added to the list : faster loading.
- some work on the display options when loading to load faster, but there is still work to do (looks like it is placing, then resizing, then creating the new Windows, but if there are too much the number of columns changes, and you need to place and resize again... long!!)
- in createWidgets, anytime new vtk are added, the synchronization is set to Delayed, to avoid frozing the software if there are too much points to process.