Previously was done per type (i.e. for each type delete loader and then
the data), wasn't usable when the loader stored different resource type
(which is the only case where it is useful).
It prevents unwanted implicit conversions from e.g. nullptr to Camera,
Vector2 to Physics::Point etc. By making all the constructors explicit
it is easier to routinely add the keyword to all new classes instead of
thinking about cases when to add and when not to.
Now whole Magnum, Magnum::SceneGraph and Magnum::Math namespaces are
fully documented -- each class has at least "getting started"
documentation, larger modules are documented on separate pages.