Support multiple documentation items with the same name

The architecture of doxylink was based on a name:item dict, which
doesn't support storing multiple items with the same name. This lead to
several workarounds:
* To support overloaded functions, function items were turned to
  name:FunctionList;
* To prevent friend declarations from conflicting with classes, friends
  were skipped.

Issues remained nevertheless: C++ allows declaring a class, a function,
and a variable with the same name:

    int a;
    void a();
    class a;

In addition, a common pattern with Qt is to declare properties with the
same name as the getter (Qt itself does this all the time):

    class Foo: public QObject
    {
        Q_OBJECT
	Q_PROPERTY(int bar READ bar)
    public:
        int bar() const;
    };

In all the above examples, doxylink was just crashing when parsing the
Doxygen index.

Instead of adding new exceptions, this commit changes the architecture
to allow multiple symbols with the same name. Duplicate names may still
raise an error, but only when one tries to refer to such an entry. This
means that all other symbols in the project can now be used instead of
failing during initialisation.

With this PR, it should be possible to create a domain in the future,
supporting much of the same syntax as the built-in [cpp
domain](https://www.sphinx-doc.org/en/master/usage/domains/cpp.html#cross-referencing)

Supersedes #31.
Closes #54.