[1.6] Use shared_ptr for library entities

[IvanSavenko]

This change removes manual memory management from VCMI_Lib classes (handlers) and replaces raw pointers with shared_ptr.
Effectively independent part of #3997.

Originally intended to use unique_ptr instead, but that turned out to be more complex & resulted in less readable code. Kind of wish there was smart pointer which defines only one 'owner' and possible 'borrowers' (other than raw pointers or weak_ptr which can be easily promoted to shared_ptr).

List of changes:

• All classes that derive from IHandlerBase (hero/creature/town/etc handlers) now use shared_ptr as storage for objects
• Removed unused and non-functional since 1.4 'update entity' logic from VLC classes
• Removed some cases where game would get non-const pointer to VLC objects
• Game entities are now non-copyable, to prevent subtle operations on copies
• Reduced direct access to VLC in favor of identifiers promotion to objects