Use C++11 final and override keywords for overwriting virtual methods

[Ithanil]

C++11 allow two new keywords to make clearer to both user/programmer and compiler, if a virtual class method is overriding a virtual base class method (override keyword) or if a non-virtual class method is the final implementation of a virtual base class method (final keyword).

To do:

• Add override keyword for all virtual (i.e. not final) derived implementations of virtual base class methods
• Add final keyword for all non-virtual (i.e. final) derived implementations of virtual base class methods

Benefit:

• more readable and explicit
• less error prone (e.g. compiler can check if we are actually overriding a base class method)
• faster compile times maybe?

[Ithanil]

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