Fixed on_erase, on_linkage_changed bug, added possibility to disable cloning,...

Fixed on_erase, on_linkage_changed bug, added possibility to disable cloning, added initialize_clone for clarity

- Fixed sneaky bug that added empty set of handlers for either on_erase
or on_linkage events that later prevented ContextItem destruction due to
m_handlers were still containing an empty set of hanlders.
- It is now possible to disable cloning by overriding clone method that
returns a nullptr. Objects with such clone method defined won't be
copied during copy operation.
- Added initialize_clone method for clarity. By default the
initialize_clone() method calls initalize() method to mimic the previous
behaviour, e.g. initialize() is called after all objects has been
copied. If user needs a better control and clarity, they can override
initialize_clone() method.