Make Doxygen debug operator command usable with enums.
Audio: completed documentation review.
DebugTools: completed documentation review.
Math: completed documentation review.
MeshTools: completed documentation review.
Platform: completed documentation review.

Fixed Doxygen errors, updated links to GLUT/SDL2 bootstrap, unified
MAGNUM_*_APPLICATION() macro argument naming.
Primitives: completed documentation review.
SceneGraph: completed documentation review.

Worked around all Doxygen issues (they were for deprecated functionality
only, so shouldn't be much of a problem anyway).
Shaders: completed documentation review.
Shapes: completed documentation review.
Text: completed documentation review.

Added missing documentation and license header to deprecated header.
TextureTools: completed documentation review.
Trade: completed documentation review.
Completed documentation review in root namespace.