SFINAE check for operators

[skident]

Consider enabling operators in a Property class if those operators are available in a wrapped type.
For example std::vector and std::map have an operator[] and if there is no specialization of a Property class for those types, you cannot use operator[] directly, but need to get .value first.

Without specialization

 auto& title = config.app->main_menu.value[0].title;

With specialization

 auto& title = config.app->main_menu[0].title;