[research] Implement ThemableMixin similar to Polymer version

[Serhii Kulykov]

Starting from lit/lit-element#470,finalize method is now protected and is used to collect styles from the prototype chain. We can try to use that point to collect additional style modules for shadow parts, roughly the same as ThemableMixin works. Also, the _getUniqueStyles method looks as what we could override (but that one is private and we should use it carefully).

[Serhii Kulykov]

Done: https://github.com/vaadin/component-mixins/tree/master/packages/themable-element