factory always return a new instance. issue #6 fixed

I don't understand why i wrote those code. wtf
portaudio: disable the code i haven't understand

those code are from QMPlay2.
portaudio should be the final class inherits AudioOutput, so it's
private data class should be in cpp.

TODO: more complete api for AVOutput, the final class's header should
not be exist if we use factory
gcc version check macro
examples: add graphicsitem example