Documentation tweaks.

Mostly move widgets classes into the QtMultimediaWidgets module, but
also mark a few mostly internal playlist classes as \internal.  A few other
tweaks.

Change-Id: If0e376410bf2187cafb24f583108c6beb7b910a9
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/multimedia/radio/qradiotuner.cpp

@@ -87,7 +87,7 @@ namespace
     You can change between the frequency bands using setBand() however it is recommended
     that you check to make sure the band is available first using isBandSupported().
 
-    \sa {Radio Overview}
+    \sa QRadioData, {Radio Overview}
 */