Add View class

[almarklein]

Relates to #492 and #495

This adds a new View class that replaces the Viewport class and Display/show functionality. Its a bit like a combination of both. The idea is that it automates all the plumbing to connect everything together for most use-cases.

Later we will make the renderer a bit more low-level by removing events; the viewport will take that role instead. See #492. One can also easily create subplots with this. For now this will be rather inefficient, but it will become efficient again when we refactor the renderer to have a predefined size and not wrapping a canvas anymore (#492).

The idea is that with this, ppl can already move to the new api.