qmllint: Can't set maxwarnings (or other arguments) from CMake

There's currently not option to modify the options passed to qmllint (except for providing a qmllint.ini file, but that's not necessarily part of a repo).

We should probably provide a way to explicitly set arguments for qmllint via a target option (and potentially globally on all_qmllint).

We need to be careful how this interacts with a user's qmllint.ini file though.