Add GUI for --log-level to Build Settings

When working with complex QBS-based projects that include custom modules it is sometimes difficult to track a specific issue in the project-files/modules. It would be very handy if one could compile with settings like '--log-level debug' in those cases.

Currently this requires a separate installation of the same QBS-version since qbs.exe is not included in the Qt Creator package. Additionaly it is difficult to replicate the build environment of the integrated build on the command line.

Therefore I would suggest to add a combo-box 'Build Verbosity' that allows to set a custom log-level.