gwin32: Add GWin32Dispatcher for message pump integration

Integrating an external main loop with the Windows message pump is far from trivial. This is primarily because DispatchMessage() may block, leaving GMainContext to starve indefinitely. The most common example is in a UI application while resizing a toplevel window. Windows will in this case, as part of its message handling when the resize is started, start a nested message pump which processes messages until the user chooses to let go in order to accept the new window size.