The renderer will provide `resize`, `reposition` and `minSize` (for
the `Scrollable` `Element`) with a read-only pointer to the model of
the application (similar to how it is already done for `handle` and
`content`). Every interface function now has the same data that it can
each use for implementing its corresponding task based on local and
shared variables through the element instance and model pointer.
Now the `App` contains a state which is a user-defined `struct` which
is passed to the `handle` and `contents` callbacks for `Container`'s and
`Element`'s. Built-in `Element`'s shall not access the `App.Model` and
should therefore never cause any side-effects.
User-defined events shall be used to act as *messages* to cause
potential side-effects for the model. This is the reason why only
the `handle` callback has a non-const pointer to the `App.Model`. The
`contents` callback can only access the `App.Model` read-only to use for
generating the *view* (in context of the elm architecture).
Renamed `Container.contents` to `Container.content` to be in line with
the corresponding `Element` function name. This has also been done for
the properties structs used by the `Container`.
The namespace shall also be used for testing the rendering of
`Container`s and `Element`s (including the `Scrollable` element).
The testing renderer currently is a striped down version of the double
buffered render without the secondary buffer and the flushing to stdout.
The internal `Cell` slice (the *screen*) is used for equality checks.
The testing namespace shall provide a way to describe the expected
`Cell` slices that should be validated against.
