Added Buffer::size(), Buffer::data() and Buffer::subData() queries.
Don't build magnum-info by default.

Causes more pain than solves (i.e. all platforms other than
non-embedded Linux fail because no windowless application exists yet).
Added TODO.
Math: deinlined heavy functions, removed redundant `inline`.

Also fixed unary RectangularMatrix::operator-() and Vector::operator-()
documentation (was stating that the operation is done in-place, which is
impossible.
*Tools: deinlined heavy functions, removed redundant `inline`.
Platform: inline/virtual optimizations.

Removed redundant `inline` keyword, changed public (pure) virtual
destructors to protected nonvirtual ones.
Shaders: removed redundant `inline` keyword.
Shapes: removed redundant `inline` keyword.
Trade: removed redundant `inline` keyword.
Text: removed redundant `inline` keyword.
SceneGraph: removed redundant `inline` keyword.
Deinlined heavy functions and removed redundant `inline` everywhere else.