wasm: add local file access private API

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).

Change-Id: I4e28baa032d7c3cd63241465f0ae55efd219a05b
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>