qmlglsink: add win32 support

qmlglsink: add win32 support

The current state of c++ ABI's on Window's and Gst's/Qt's conflicting
mingw builds means that we cannot use mingw for building the qt plugin.

Instead, a qmake .pro file is provided that is expected to be used with the
msvc binaries provided by Qt like so:

(with the PATH environment variable containing the path to the qt biniaries
and PKG_CONFIG_PATH containing the path to GStreamer modules)
cd /path/to/sources/gst-plugins-bad/ext/qt
qmake -tp vc

Then open the resulting VS project and build the library.  Then

cp debug/libgstqtsink.dll /path/to/prefix/lib/gstreamer-1.0/libgstqtsink.cll

https://bugzilla.gnome.org/show_bug.cgi?id=761260