ColladaImporter: proper on-demand scene and object parsing.

Until now whole scene tree was parsed at once with several issues:

 * Error while parsing object might be emitted when parsing totally
   different object.
 * The objects were parsed once and then returned as pointers, thus the
   user couldn't delete them.
 * More data than strictly necessary was processed.
 * The code was an entangled mess.
ColladaImporter: made internal classes private.

No need to have them in header, allows to significantly reduce
includes.
TgaImporter, TgaImageConverter: adapted to Magnum changes.

It looks like I forgot to `delete` three times in the tests. It just
proves that the previous API was flawed (or unusable for people spoilt
with RAII like me).
PngImporter, JpegImporter: adapted to Magnum changes.

Unsurprisingly, I didn't call `delete` here at all.
MagnumFont[Converter]: adapted to Magnum changes.

Forgot to call `delete` here too.
ColladaImporter: adapted to Magnum changes.

No deletes forgotten here, yay to me!
ColladaImporter: adapted to Magnum changes.