Access to the local file system is restricted by the
Web sandbox, and a separate API an implementation is
needed to facilitate this for Qt applications.
This adds a private asynchronous callback-based C++
API for opening a file dialog and reading file content.
The implementation uses a file input html element to
show a file dialog, and then the uses the native File
and FileReader APIs to read the selected file(s).
Reviewed-by: Lorn Potter <firstname.lastname@example.org>