Add the ability to disable the fallback scanner

QtCreator runs a fallback scanner if project resolution via CMake fails. I want the option to disable the fallback scanner entirely as it's not useful and it blocks me from quickly re-run CMake with modified settings. Instead, it would be better to simply show the project file tree so that the user has a chance to modify the CMakeUserPreset.json for instance.

Steps to reproduce:

1. Create a CMake project folder that contains vcpkg as a submodule. This contains a gazillion files.
2. Open the CMake project in QtCreator and ensure that project resolution fails
3. QtCreator will run the fallback scanner. That takes minutes.
4. Try to abort the fallback scanner. It takes up to 10-20s for the scanner to really abort.

Suggested behavior:

1. User opens a CMake project
2. Project resolution fails
3. QtCreator shows the unfiltered file tree of the project folder
4. User may change a CMake user preset or adjust the kit or even another file in the project and is able to immediately re-run CMake

Background:

If I opened a project via a CMake file, it is very likely that I want to explore the project via CMake and only CMake. I don't want any intelligent fallback mechanism as it takes my precious time.