refactor: move "internalpayerbackend", which is not a backend, to src/audio

[shenlebantongying]

I think this is a good time to switch default to Qt multimedia.

Then I find this weird warning. Why the hell changing the default require changing more code?

goldendict-ng/src/audio/audioplayerfactory.cc

Lines 52 to 54 in c7d0feb

	#ifdef MAKE_FFMPEG_PLAYER
	Q_ASSERT( Config::InternalPlayerBackend::defaultBackend().isFfmpeg()
	&& "Adjust the code below after changing the default backend." );

Then I found this unusual thing in the middle of config.

It is called as "backend" and can be acts as "backend", but it doesn't work as a "backend" (because it invokes other backends).

Anyway, let's move it out from the big config.cc.

git blame is not useful here. The commit message is just useless blurps 9aa3c44

[sonarcloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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