Removing XML::NodeEventVector

XML::NodeEventVector was a C-style way of emulating virtual functions which doesn't need to exist anymore. this paired with XML::Node::addListener was used to GC allocate observers while not having to care about their lifetimes.

This was accomplished by inheriting from XML::NodeObserver in classes which handled the code for adding and removing observers in most cases. In some cases (see NodeData in src/widgets/sp-xmlview-tree.cpp), a pointer to an XML::NodeObserver is used to provide a more dynamic approach.