Track screen size, viewport size, screen pixel bits and language

Screen size, viewport size, screen pixel bits and language are quire interesting information for a Web app developer. The information aren't that important for dart:io applications, therefore I only implemented it for the HTML version.

I would say that screen size, bit depth and language are constant parameters for a session, we could also initialize them in the constructor of ```AnalyticsHtml```. The client/viewport size can change.

*Tests:* I thought about adding tests, but there is no infastrature to test the HTML version yet.