SceneGraph: don't use virtual calls when setting transformations.

Until now, all calls to e.g. `Object::translate()` were virtual, which
is _very_ bad for performance. The virtual call is only needed when
setting the transformation via some interface, e.g.
`AbstractTranslationRotation3D`, as the caller doesn't know which
transformation implementation is used.

Now all public-facing transformation methods are inline non-virtual
functions, which are in most cases calling directly the transformation
implementation. In `Abstract*` transformation interfaces these functions
call private virtual `do*()` implementations, which are (re)implemented
in subclasses, but aren't used anywhere except when transforming
directly through the `Abstract*` interfaces. This should have good
impact on performance when doing many transformations in every frame
(although I can't verify it anywhere, as I don't have any significantly
large animated demo). Except of course when doing it through the virtual
interfaces.

As the public-facing transformation methods are now non-virtual, there
are now no "covariant return" issues and they can now return proper
`Object<*Transformation*>` type instead of just `*Transformation*`,
which makes full non-WTF method chaining possible:

    Object2D* obj2;
    obj2->translate({0.5f, -1.0f})           // Transformation method
        ->setParentKeepTransformation(obj1); // Object method

Or even this:

    Object2D* obj = (new Object2D)->rotate(-15.0_degf);
Documentation fixes.
SceneGraph: make public Object API non-virtual too.

Calling functions directly on Object will result in non-virtual calls,
calling functions on AbstractObject will result in virtual calls. Also
removed now unneeded "hacks" like `sceneObject()` and properly hiding
unsafe functions taking `AbstractObject*` with safe functions taking
`Object*`. Also hide `isScene()` from public documentation, as it is
sort-of hack too.
SceneGraph: typedef MatrixType to avoid those horrible declarations.
Math: fix compilation with Clang.

It seems that with Clang you cannot split declaration and definition of
`constexpr` function. These should be as short as possible anyway, thus
it is non-issue.
SceneGraph: extracted CachedTransformation[s] enum out of AbstractFeature.

Less typing, less confusion.