Support a way to easily set import paths for QML engine & tooling

For deployment and installation use cases, it is necessary to add additional import paths to the QML engine, as QML modules might not end up in any of the default import paths (which might not even be writable).

A convenient way would be to have a file allowing to set a list of import paths. Those would either amend or replace the existing import path locations.

We already have a somewhat similar mechanism in qt.conf via QML(2)_IMPORT_PATH. However, that only allows adding one new path, and also has the issue that the default (file system) import path of the Qt modules would be gone.

However, it might be possible to extend qt.conf to support this use-case, too.

In order to avoid issues with tooling, the mechanism needs to be understood by both the engine and our tooling (compiler, linter, etc.).