Crash due to QtOpenGLDummyWindow window class being unregistered prematurely

Calling createDummyGLWindow() (in qwindowsglcontext.cpp) will lead to a call to RegisterClassEx with the class name "QtOpenGLDummyWindow" in QWindowsContext::registerWindowClass().

In our case this causes a crash in the following scenario:
When running three (different) audio plugins (VST) that use Qt in a host environment like e.g. Cubase on Windows all of them will register the "QtOpenGLDummyWindow" window class (with the address of the Windows process added as a suffix in case the class name has already been registered, see QWindowsContext::registerWindowClass()).
When closing the user interfaces of the plugins, then removing the second plugin and then opening the user interface of the third plugin it will crash because the window class "QtOpenGLDummyWindow+processAddress" will have been unregistered already (by the second plugin).