Draft: Make Workspace process output changes when Platform::screensQueried is emitted

Some platforms remove all outputs then add new outputs. That can result in a case where the platform has no output. Window management code in kwin is not designed for that case, it will most likely crash.

Draft as I don't think it's a good solution in long term. If no adequate solution is found in nearby future, we could merge this patch until the issue gets properly fixed.