Qbs: add explicit resolve method

[ABBAPOH]

Changelog: Feature: Added resolve() method to the Qbs toolchain. Docs: Omit

Most tools have a separate configuration step and Qbs is no different. It's strange that the API doesn't provide a separate method for this. Add it now so that users can understand better what went wrong during which stage.

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• [ ] I've opened another PR in the Conan docs repo to the develop branch, documenting this one.