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`.
This also means that currently the dynamic resizing through the app's
detached thread is not working, as it cannot send size updates. The
examples have been overhauled to still implement intermediate mode
applications accordingly.
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.
Remove examples, add description for design goals in README.md and
apply re-names and naming changes accordingly for the project structure.
Implement a flat hierachry, as the library shall remain pretty simple.
This removes the App.Renderer.Buffered, which was not improving on the
direct renderer, which has a simpler implementation and was keept for
this regard.
The App.Renderer is used for the new `Layout.render` method. Each layout
renders itself now with corresponding renderers which might only update
parts of the screen, etc.
