Don't let qmllint warnings fail the build

If the `all_qmllint` target is built, the build will be considered to have failed when a qmllint warning is encountered. One example consequence is that Qt Creator will not launch the startup application even if it is perfectly runnable.

Suggestion: Don't treat qmllint warnings as a build failure by default. (Perhaps reserve that for the Error level, or support a qmllint equivalent of -Werror)