Prevent call to gst_element_set_state from a streaming thread

it's possible for gstreamer to use a stream thread to emit all
sync-message signal, but gst_element_set_state can not be called from a
mainloop other than main thread, here we use QObject::invokeMethod to
make the call happen in main thread.

BUG: 339511
REVIEW: 120425
FIXED-IN: 4.8.1