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/wmf/wmf.json

@@ -0,0 +1,3 @@
+{
+    "Keys": ["org.qt-project.qt.mediaplayer"]
+}