Math: doc++
Math: complex number multiplication.
Math: complex number dot product, length and normalization.
Math: construct identity transformation by default.

Square matrices already had that, (dual) quaternions too, making that
the default also with complex numbers. Updated the documentation to
reflect that.
Math: conjugation and inversion of complex numbers.

Also updated related Quaternion and DualQuaternion documentation.
Math: angle between two complex numbers.

Also crosslinked the documentation with similar functions in Vector and
Quaternion.
Math: rotation complex number.
Math: converting complex number to rotation matrix.
Math: converting Vector2 to Complex and back.
Math: transforming vectors with complex numbers.
Math: forgot to add Complex to forward-declaration header.
Brought Complex and DualQuaternion to Magnum namespace.
MeshTools: reworked and updated transform*().

Also added just-to-be-sure unit test.