These modification are not necessarily directly related to the
containing PR, I merely saw them while reviewing the containing PR.

Do the selection in mapStateToProps so the container itself doesn't need to
receive all the props that each overlay needs.

Each overlay is responsible for fetching their own props and for providing a
"needsDisplay" static method wich will be called with the full redux state and
should return true if the overlay needs displaying.

Also eliminate duplicated state keeping: the connection and conference error
states can be fetched from their respective base features.

Prepare for React 16.

In this case makes more sense to have overlay frame included in every overlay instead
of abstract class that implements the overlay frame and have to be extended by every
overlay. In addition, mapStateToProps isn't working well with inheritance.

Split language detectors to be web/native dependent. Take care of strings that contain html.