Updated plugins to use new plugin architecture

Updated plugins to use new plugin architecture

Changed QMediaPluginLoader to use QFactoryLoader instead of QPluginLoader
and used metadata to get keys.
Removed QAudioPluginLoader and changed audio classes to use instead use
QMediaPluginLoader.
The plugins must include the Q_PLUGIN_METADATA macro, and no longer use
the Q_PLUGIN_EXPORT/Q_PLUGIN_EXPORT2 macros.
A json file has been added for each plugin which can contain metadata
which is available to the plugin loader before the plugin is actually
loaded, and is used to read the keys for the plugin, e.g. supported
services.
QFactoryInterface will be deprecated and has been removed from all
plugins.

Change-Id: I035b82f9c9c65717bebf704d560ea8f891df21da
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>