Remove deprecated features in experimental code.
Use deprecated attribute of supported compilers.

This replaces the QL_DISABLE_DEPRECATED mechanism that conditionally removes
the features and causes the compiler itself to emit warnings if the features
are used.  The user can enable or disable the warnings by the means provided
by the compiler.
Rename deprecated list in generated docs.
Enable deprecated access to protected data members.

Protection level can't be switched with compiler attribute.
Access to those data members is one of the several things we'll fix
when we rethink the library.
Remove deprecation comment.

The suggested alternative needs a number of inputs more than the
class that was being deprecated.
Reworked deprecation mechanism.

Now we rely on compiler facilities to issue warnings when deprecated
features are used.