doc: added Doxygen layout file.

Will be used to tweak the documentation output even more.
doc: show include for each class.

Had to explicitly disable showing includes for files. Useless stuff.
doc: don't show files used to generate the documentation.

Also useless.
doc: updated directory documentation.

Noting what dir is part of which library.
package: adapted to Corrade Jenkins CI changes.

Namely using deprecated/non-deprecated Corrade build.
SceneGraph: fixed compilation of Drawable on GCC 4.6.

This error was present since the very beginning but it didn't appear
until we were explicitly instantiating the Drawable class.
SceneGraph: fix documentation of DrawableGroup aliases.
Shaders: don't use deprecated header in tests.
Fixed OpenGL ES 2.0 and 3.0 build.