Use override keyword where applicable

[feliwir]

The override keyword was introduced in C++11 and allows to get compilation errors when a function does not override something from the base class. See https://en.cppreference.com/w/cpp/keyword/override

• replace virtual keyword in derived classes
• add override keyword where it was missing before

[autious]

Does the removal of the virtual keyword replacing it with override mean it'll still be in the vtable in case of polymorphism?

[feliwir]

Yes, it should still be inside the vtable. There's also the new final keyword in C++11, which will impact how the vtable is generated (allowing for more optimizations). However we don't use that yet