dual head support (SDL_VIDEO_FULLSCREEN_HEAD) for Qt and SDL2

This is dual monitor support, both for SDL and Qt. The display can now be specified with `SDL_VIDEO_FULLSCREEN_HEAD`, where `0` is the primary monitor and `1` the secondary.

It is compliant with SDL1.2. For use of SDL2 please compile with `-DUSESDL2`. The SDL2 code is partly based on the proposal by g2p: https://github.com/beaumanvienna/native/blob/retrorig-pl1/base/PCMain.cpp#L570
Without `-DUSESDL2`, the SDL changes are neutral and only the Qt application will react on `SDL_VIDEO_FULLSCREEN_HEAD`.

Both the changes for SDL and Qt are non intrusive and have no effects on ppsspp elsewhere, except a message "sdl1.2 code" when compiling PCMain.cpp w/o SDL2.

Please merge the changes from this PR together with the changes in the [main ppsspp repository](https://github.com/hrydgard/ppsspp/pull/6937).

This PR has been discussed [here](https://github.com/hrydgard/ppsspp/pull/5201#issuecomment-56337690).
you could just use this with Qt:
int getDisplayNumber() {
    return QProcessEnvironment::systemEnvironment().value("SDL_VIDEO_FULLSCREEN_HEAD", "0").toInt();
}