Add support for ahead-of-time generation of QML type caches

The QML engine (as of Qt 5.8) generates cache files on the disk at run-time that contain data structures and generated code that make it possible to avoid the length type compilation step the next time the .qml/.js file is loaded.

We want to be able to generate these cache files at application build time, in regular build environments as well as when cross-compiling.