Improve API architecture

[wzieba]

• Make internal everything what’s not needed to be exposed to clients.
• Check if API make sense and covers needed cases (at least for Woo Android for now).
• Check UI customisation possibilities (colors?).

[wzieba]

We should provide API for library consumers to share sources of the images. Ideally, it should be an async one with availability to share state of the source loading (loading, success, failure).