Adapted to Magnum changes.
TgaImporter: parsing the image on call to image2D().

Solves the problem about image deletion, the image is imported again on
every request and the user is responsible for its deletion. Now also
asserting that the file is opened and that the image ID is zero.
ColladaImporter: assert that document is opened and ID is in bounds.
ColladaImporter: don't act as data storage & garbage collector.

All data except objects and scenes are imported again on every request
and the user is responsible for deletion.
TgaImporter: minor code cleanup, blind ES compilation fix.
TgaImporter: support for grayscale images, updated error messages.

The messages now print fully qualified function name.
TgaImporter: moved TGA header structure to separate file.

Will be used elsewhere.
Adapted to Magnum changes.
Updated FindMagnum.cmake from Magnum repository.
TgaImporter: test also file opening.
New plugin TgaImageConverter.