Remove the last remainders of the old plugin system

Port the last two currently unused plugins (v4l and wmf) over to the new plugin
format.

Fix documentation that still mentioned Q_EXPORT_PLUGIN.

Change-Id: I18200dd792e8973687e92180fc41d11395b35132
Reviewed-by: Jonas Rabbe <jonas.rabbe@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/plugins/v4l/v4lserviceplugin.h

@@ -51,8 +51,8 @@ class V4LServicePlugin : public QMediaServiceProviderPlugin, public QMediaServic
 {
     Q_OBJECT
     Q_INTERFACES(QMediaServiceSupportedDevicesInterface)
+    Q_PLUGIN_METADATA(IID "org.qt-project.qt.mediaserviceproviderfactory/5.0" FILE "v4l.json")
 public:
-    QStringList keys() const;
     QMediaService* create(QString const& key);
     void release(QMediaService *service);