Each shader now has sample image, example mesh configuration and example
rendering setup. Also properly documented all attribute types and made
introductory chapter for whole Shaders namespace.
Direct access to list of children is now provided through
Object::children(), list of features is provided in
AbstractObject::features(). In most cases the range-based-for is good
enough, the previousSibling()/nextSibling() and
previousFeature()/nextFeature() functions are for the cases where user
needs more flexibility.
Because everything that was previously done using firstChild() etc. can
be now done also with children().first() etc., there would be more than
one way to do the same thing. Thus the old functions are now marked as
deprecated and will be removed in some future release.
Main change is that features take reference to containing object instead
of pointer, as the feature must always belong to some object. Feature
groups now return references to features and features return reference
to containing object, as these cannot be null.
Passing `*this` to AbstractFeature (and Camera[23]D) constructor might
now clash with deleted copy constructor, added templated constructor to
catch and resolve these ambiguous cases.
Makes some cases less consistent (and some convenience shortcuts
impossible), but goes well with the attitude "don't use pointer when it
can't be null".