Evidently gcc doesn't recognize inline forward declaration, but clang does.
I wonder who is right?
This was previously addressed by adding an #include. Instead, this patch
adds a standalone forward declaration of the QXmlStreamReader/Writer classes.
This has been tested and verified to compile with gcc and clang.
Calling openConnection will return an open connection or nullptr.
Deleting the connection will close it.
SerialPort now uses this under the hood, while still presenting
the QSerialPort-compatible interface.
This initial commit is designed to change as little existing code as possible.
Once regression tests are in place that can play back previously recorded
data, we can move on to more significant changes.
