Introduce the concept of an "active" bufferview

Introduce the concept of an "active" bufferview

For things like keyboard navigation, we need to mark a given bufferview as the active
one, so concepts like "next/previous channel" have something to refer to. We now mark
the active bufferview by a little dot. The user can switch through the visible views
using the "Forward" and "Back" standard keys, which usually map to Alt + Right/Left.

No (re)storing of the active view yet.