Trade: full scene conversion support in AbstractSceneConverter.

Wasn't really possible to split this into multiple commits, so here's
the whole thing including delegation from and to the single-mesh APIs.
What's not done here and postponed for later is:

 - an ability to feed the whole importer to it, filtering away data that
   aren't supported by the converter
 - corresponding changes in AbstractImageConverter, where it would now
   primarily accept ImageData to future-proof for arbitrary extra
   key/value data

doc/changelog.dox

@@ -285,6 +285,9 @@ See also:
 -   New @ref Trade::SkinData class and @ref Trade::AbstractImporter::skin2D() /
     @ref Trade::AbstractImporter::skin3D() family of APIs for skin import, as
     well as support in @ref Trade::AnySceneImporter "AnySceneImporter"
+-   The @ref Trade::AbstractSceneConverter plugin interface gained support for
+    batch conversion of whole scenes --- meshes, hierarchies, materials,
+    textures, animations and other data
 -   1D and 3D image support in @ref Trade::AbstractImageConverter
 -   @ref Trade::LightData got extended to support light attenuation and range
     parameters as well and spot light inner and outer angle

src/MagnumPlugins/AnySceneConverter/AnySceneConverter.cpp

@@ -98,4 +98,4 @@ bool AnySceneConverter::doConvertToFile(const MeshData& mesh, const Containers::
 }}
 
 CORRADE_PLUGIN_REGISTER(AnySceneConverter, Magnum::Trade::AnySceneConverter,
-    "cz.mosra.magnum.Trade.AbstractSceneConverter/0.1.2")
+    "cz.mosra.magnum.Trade.AbstractSceneConverter/0.2")