Vtk3dVisualizer uses QVTKWidget (legacy) or QVTKOpenGLNativeWidget

VTK deprecated the old Qt widget QVTKWidget and added the new QVTKOpenGLNativeWidget.

This PR allows the vtk3dVisualizer to work with both widgets. The new widget is the standard for VTK >= 8.0. For VTK 8.x it is possible to choose between both widget types, whereas the new one is the standard. The legacy version can be enabled by the CMake option VTK3DVISUALIZER_FORCE_LEGACY_VTK.

Together with this change, this PR also brings further enhancements of the widget, e.g. documentation improvements of the point pick event and other smaller bugfixes.

Please consider, that the current version of point cloud library in Ubuntu (at least 20.04) is not compiled against Qt, therefore the vtk3dvisualizer will not work there with the standard version of PCL. However you need to compile PCL there by yourself and enable the Qt support.