Make Qt Quick Designer launchable in "stand-alone mode" (e.g. from Qt 3D Studio)

Qt Quick Designer, which is a component in Qt Creator, shall be launchable as a semmingly stand-alone application. Qt Creator needs a kind of design-only mode (selectable e.g. via command line parameter). All other components that have nothing to do with Qt Quick designing should be either not loaded or be hidden.

Bigger picture: One of the most important mid term plans for Qt is to unify the 2D and 3D stories. On the framework side, we will see a deep integration of 3D Studio runtime with Qt 3D and following that a merge of (Qt 3D Studio + Qt 3D) with Qt Quick.

Also on the tooling side, we are aiming for a unification. We have Qt 3D Studio on one side and Qt Quick Designer on the other. From a user perspective, they should co-operate as seamlessly as possible. The plan is a shallower integration as in the framework case. Qt Quick Designer should be launchable from 3D Studio when a "2D form canvas" in the 3D space is selected for editing. The launched Qt Quick Designer should then not have "that IDE stuff" around it, which is intimidating to non-developers.

The "design mode" of Qt Creator should idealy be a run-time (start-time?) option, rather than a compilation flags.