Potential memory leak due to missing virtual destructor in AnchorVertex base class

AnchorVertex is subclassed by AnchorVertexPair, which adds non-POD data members, resulting in the compiler generating a default destructor.

AnchorVertexPair objects are allocated with operator new and upcast when placed into QList<AnchorVertex*>, for instance in calls to replaceVertex. Deletion of those instances is then done via qDeleteAll on the lists, or otherwise primarily by deleting AnchorVertex pointers. There is only a single case where a AnchorVertexPair* is deleted, and no case where the type of the instance is checked (via the m_type member) before deleting instances.

This makes it very likely that there are memory leaks, as deleting an AnchorVertexPair instance through a AnchorVertex* will not call the generated ~AnchorVertexPair destructor.

Coverity ID 218707