QtQuick startup time needs to be improved; cache compiled QML between runs

To improve application startup time, the QML engine needs to allow the compiled version of the QML code to be cached between runs. i.e. caching of .qml files in a fast, mmap()'able binary format on disk.

This should be a cache of the post-compile stage data, in a form that requires minimal tweaking after mmap()'ing so that the memory can be efficiently shared between different processes. This will require a rework of the QML internals to generate data in an appropriate format.