Implement QAsyncFile / QFileV2 / ???

As discussed at the hacking day of Contributor Summit.

This is a tentative plan - depends on outcome of QTBUG-136763.

To be implemented as a QFileDevice(/QIODevice) as a replacement / complement to QFile that returns data asynchronously, and uses the QIODevice signals consistently. The implementation should extend the QFileDevice API with new signals. For example, for an async open() call (or should we call it connect()?).

The idea of the implementation is to piggy-back off the work that will go into QTBUG-136763 and use that to read the file in full.

Potential considerations:

• maxReadBufferSize mechanism, to avoid pulling a full multi-gigabyte file into memory unless desired.
• How to request file data starting at some offset - or will this simply not be supported (e.g. you have to skip the bytes you don't care about and then read from the offset you want)
• ...