Codesign Designer, Assistant and Linguist (and qdbusviewer?) apps

The applications that are shipped with Qt, Designer, Assistant and Linguist, are currently unsigned. In principle that shouldn't be a problem - the installer is signed, and macOS still allows running unsigned applications - but unfortunatly there is a bug in Apple's Security framework that affects all unsigned, universal (intel+arm) applications.

The bugs leads to the permission dialogs, asking the user for permission for access to e.g. the Documents and Downloads folder, to re-appear everytime even after the user already gave permission, which is pretty annoying.

See QTCREATORBUG-26705 for more details.

Since this issue doesn't appear with signed applications, and signed applications are nicer citizens on macOS anyhow, we should sign the applications that we ship with Qt.