Math: base class for units.
Math: type-safe implementation for degrees and radians.

Next few commits will add requirement for "strongly typed" angles in all
function parameters, e.g.:

    Matrix3::rotation(24.0_degf);
    Math::sin(1.047_radf);

The purpose is to make angle entering less error-prone, e.g. not passing
degrees when radians should be etc.
Brought Math::Deg/Rad and related operators to Magnum namespace.
Math: use Rad for Vector::angle().
Math: using Rad in Matrix*::rotation*().

Also updated dependent functions and tests.
Math: using Rad for Matrix4::perspectiveProjection().

Also updated dependent functions and tests.
Math: using Rad for Quaternion and DualQuaternion functions.
Using Deg for Color hue.
Math: removed old deg()/rad() helpers.
Math: minor code cleanup.
Math: added sin(), cos() and tan() accepting either Rad or Deg.
Platform: don't allow SDL2 to redefine main().
Text: don't use HarfBuzz ICU functions.

Apparently not needed at all.
Platform: doc++