Add the audio decoder variant of QMediaPlayer.

Rather than the probe based way of doing it.  Initially private.

Change-Id: I30005f8da22f6451cb9de8eb3f0e193838d48c93
Reviewed-by: Ling Hu <ling.hu@nokia.com>
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>

src/multimedia/controls/controls.pri

@@ -35,6 +35,9 @@ PUBLIC_HEADERS += \
     controls/qmediaaudioprobecontrol.h \
     controls/qmediavideoprobecontrol.h
 
+PRIVATE_HEADERS += \
+    controls/qaudiodecodercontrol_p.h
+
 SOURCES += \
     controls/qaudioencodercontrol.cpp \
     controls/qcameracapturebufferformatcontrol.cpp \
@@ -67,4 +70,5 @@ SOURCES += \
     controls/qvideoencodercontrol.cpp \
     controls/qvideorenderercontrol.cpp \
     controls/qmediaaudioprobecontrol.cpp \
-    controls/qmediavideoprobecontrol.cpp
+    controls/qmediavideoprobecontrol.cpp \
+    controls/qaudiodecodercontrol_p.cpp