Better handling of import project clash

If you were starting now, Tasker would be much better, with namespaces to avoid clashes of global variables, task names etc. We are not starting now, so have to manage with what we have. If a Tasker project is imported, and one of its tasks already exists in an existing project, the task is not imported. This is a good safe default. But a warning popup to tell the user that this had happened would be very useful. A task in the new project might use perform task intending to run one of the author's task and instead run some other task causing who knows what problems.