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.
Can you move the constant values to the constructor of AnalyticsHtml (and set those ones using `setSessionValue`)?