wasm: add public API for local file access

The web sandbox restricts access to the local file system,
which means Qt needs new public API to provide such
access to applications.

This adds a new static, asynchornous getOpenFileContent()
function to QFileDialog, which will show a file dialog,
read the file contents once a file has been selected, and
finally call the user-provided callback with the selected
file name and file data.

  auto fileReady = [](const QString &fileName, const QByteArray &fileContents) {
    // Process file
  }
  QFileDialog::getOpenFileContent("*.*", fileReady);

This API can be expanded in at least two ways in the
future: allow reading multiple files, and allow file
streaming by returning a QOIDevice.

Change-Id: I011b92fbcf21f0696604e66b7f9eb265c1a07aaa
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>