Fixed variable name in FindMagnumPlugins
modules: make FindMagnumPlugins.cmake actually working.

Broken since f3f2fc8b02d19e92402849e6a2b9b72404408d3b and never tested
since. Sigh. Thanks a lot to @wivlaro for the hint.
modules: set MAGNUMPLUGINS_INCLUDE_DIRS only on deprecated build.

Also mention that properly in the docs.
modules: updated FindMagnum.cmake.
ColladaImporter: added TODO.
doc: oops.
ColladaImporter: remove old nonsense from plugin configuration file.
Added AnyImageImporter.

Detects file type based on extension and delegates the operation to
specialized importer plugin.
ColladaImporter: use AnyImageImporter instead of TgaImageImporter.

Now virtually any image format can be supported.
Doc++
This file should not be here.
JpegImporter: minor cleanup.
PngImporter: minor cleanup.
Display all headers in project view of IDEs.

Finally makes development with QtCreator usable.
doc: mention usage of CMAKE_PREFIX_PATH.
Gentoo ebuild
Doc++, added CREDITS.md.
Updated CREDITS.md.
external: added stb_image 1.46.
New StbImageImporter plugin using stb_image.