QDS-11148 - Qt6.6 integration

Main change here is moving the "screen" element out from the "qtspinner" hierarchy, which avoids hiding it when qtspinner is hidden. Also rename it to "qtcontainer".

Other fixes:

• there is no longer a qtLoader object. We can access the instance after startup instead.
• remove old preRun workaround
• the canvas is now used only for detecting WebGL support
• call setContainerSize from onLoad
• make DesignViewer::printError() actually print the error

Closes QDS-11148