[Interfaces] Performance: prefer prefix ++/-- operators for non-primitive types.
[Interfaces] Performance: possible inefficient checking for container emptiness.
[Interfaces] Performance: Returning the result of c_str() in a function that returns std::string is slow and redundant.
[PlayList] Performance: Variable 'pos' is reassigned a value before the old one has been used.
[AddonClass] Make AddonClass really non-copyable.
[Interfaces] Initialize members in constructors.
[Interfaces] Reduce the scope of variables.
[Interfaces] Remove unused variables and reduce scope in one case.
Merge pull request #2676 from ace20022/interfaces_cleanup

[Interfaces] Clean up things found by Cppcheck