Research support conditional loading based on environment

E.g. when screen resolution boundaries change, allow developers to set up a file structure (e.g. in qmldir) that enables conditional loading of toplevel or other QML files. This would be done using qmldir local to the executable, or otherwise optimizable at compile-time, to ensure unneeded file system access and resource lookup is prevented.

This is needed to prevent forcing developers to write boilerplate or drop to C++. See also QTBUG-15381