Add ability to control the location of the QML just-in-time cache

The location of .qmlc/.jsc cache files should be controllable. Examples of issues with the current implementation:

• For release builds, we would like to keep the simple extract-archive-to-install upgrade/installation process. But we'd also like to start from a clear cache whenever upgrading software (to protect ourself against stale cache bugs in the QML just-in-time caching implementation) which means that a folder outside of the installation path needs to be deleted during software upgrades (e.g. %LOCALAPPDATA%/<COMPANY>/<APPNAME>/cache/qmlcache on Windows)

• For our development builds, we reference .qml/.js directly on the file system so we end up with .qmlc/.jsc files next to the respective .qml/.js that litter the git working tree. Even though this problem is mostly solved with .gitignore rules, it goes against the out-of-tree build philosophy.

If given a way to control the .qmlc./.jsc file locations, we'd place them in a directory somewhere relative to QCoreApplication::applicationDirPath().