turn QGstTools into a proper private module

This patch moves the QGstTools lib to a private Qt module and marks
the needed classes and functions with Q_DECL*, to improve
cross-platform abilities. It's the first step to use the GStreamer
backend on macOS and Windows platform.

Change-Id: Idc23c72bcccb52dd501e169e9dfdc227856787fa
Reviewed-by: Christian Stromme <christian.stromme@qt.io>