Deprecate the plugin architecture for asset loading

This should be done over the next release cycle.

In the original design for Qt3D we considered loading of different
file formats to be analagous to loading different image formats, and
thus it was written following the pattern of the libpng, and libtiff
format that exists in QtBase. This meant scanning the plugins
directory for a sceneformat loader plugin that supported the given
file type, and if one was found loading that and then reading the file.

However this is a poor match for the actual job and slows down the
loading of assets as the loader has to resolve a plugin before it can
actually start loading the asset.

With the support provided by Asset Importer a wide array of scene
formats is supported, and it makes most sense for new formats that
are not currently supported to be added and upstreamed to Asset
Importer.

Another consideration for the deprecation process is that our
QML bindings depend on a structured scene format. Scene-nodes,
materials and effects must be included in the scene using a set and
known schema. With the original plug-in API the idea was that plugins
would be free to do any kind of rendering or representation and thus
this schema is not able to be supported. This means that plugins
would be created that could not be rendered or would be buggy and
inconsistent in QML.

With upcoming support for animations this problem becomes even more
pressing since animations formats in asset files must also be exposed
to our QML bindings.

The QGLAbstractScene class will stay, possibly with some modifications
but the plug-in apparatus will be dismantled and thus loading of
assets via a plugin architecture will removed in favour of a direct
loading functionality, probably kept in a seperate library linked
at compile time. This should mean smaller code, faster loading and
better support for animations and future functionality.

The bezier plugin will be retained and we will look at contributing
it to Asset Importer if the format is general enough.

Since deprecation of the sceneformat plugin API is a considerable change
to our API's we will not do it lightly. So far however we have had no
contributions to the sceneformats and no-one has asked about it, or used
it, despite this tutorial existing. At present it will be marked
\bold deprecated and in our next release cycle it will be removed
altogether.