Normalize how we set top level window geometry

Prefer Window API, since for QtQuick the root view isn't so tighly
coupled with the QWindow as in QWidgets world. Hide the difference in
Window_qtwidgets.cpp, which now prefers the QWidget API whenever possible,
as that's better tested than using QWindow direclty.

src/qtcommon/Window_qt.h

@@ -28,6 +28,7 @@ public:
     QRect geometry() const override;
     void setGeometry(QRect) const override;
     bool isVisible() const override;
+    void setVisible(bool) override;
     WId handle() const override;
 
     // TODOm3: Remove