Simply rendering of QML thumbnails

We have what we call the image collector. The image collector can generate a thumbnail for each QML file.
We use the full qmlpuppet mechanism for this, including the rewriter, project storage node instances etc.
This is overkill and since we use this multi-threaded it adds quite a bit of complexity and potential errors.

This patch is required to properly handle the project storage dependency.
See: https://codereview.qt-project.org/c/qt-creator/qt-creator/+/619583

Proposal: Rewrite the image generation using a custom mode/argument of the qmlpuppet.
This mode would simply forward the QML file and import paths as arguments and/or enviroment variables. The output would be a PNG file. This way the only required context would be the file URL and import paths.
From the QDS side things would become significantly more simple.

To render the thumbnail we need to 'fork' some of the render control code. But sine we are still in the same codebase/executable we could add utility functions. We can optimise the thumbnail rendering without impacting any other paths.

From the QDS side no project, project storage, rewriter etc would be required anymore.