TgaImporter: adapted to Magnum changes.

Not storing image name inside ImageData greatly simplifies the
implementation. Also using `override` keyword where possible.
ColladaImporter: adapted to Magnum changes.

Not storing name in *Data slightly simplifies the implementation also
here, yay! Moreover it is possible to retrieve only data name without
the need to parse them fully.

Also added `override` keyword where possible and slightly cleaned up
unit tests.
ColladaImporter: don't forget to delete _everything_ on destruction.
ColladaImporter: minor cleanup, test improvement.

Using CORRADE_COMPARE() instead of CORRADE_VERIFY() where possible.