ENH : loading quicker

- The bool variable m_updateOnAttributeChanged helps not calling  the spinBox_Attribute_indexChanged() when adding every attribute during the createWidget() function
- setting the scrollArea (Display Area) visibility to false when creating widgets, then setting it to true at the end (after placing and resizing) will only render everything at the end.
- also, we set the renderAllSelection variable to false before settinf the visibility to on, to avoid unnecesary rendering.

We know have a very quick loading for multiple meshes!