Draft: Update examples so they retrieve Qt from VTK

This MR is a question more than a contribution as I am not fluent in Qt yet. Previously, examples where trying to find Qt on their own, causing possible mismatch between the Qt provided by VTK and the Qt they found. I have made some update so now examples should work with both Qt 5 & 6, as provided by VTK.

However, if in the futur an exemple requires a component not provided by VTK, I assume a find_package would be the way to go. Unfortunately, I do not know how to do it as both QT_DESIRED_VERSION and VTK_QT_VERSION seemed to raise a buggy behavior where the examples had a Qt3 not found warning appearing, and always used Qt6 at the end, ignoring these variables.

Also, Qt6 seems to required Cxx17, do we need to enforce the check at the application level ? (in these examples)