Configuration to enable scrollbar rendering for scrollable `Element`s.
Currently only the fg `Color` of the scrollbar can be configured while
the background uses the same fg `Color` but adds the emphasis `.dim` to
make it obvious what the is the actual scrollbar. In the future it might
be necessary to provide the user with more options to configure the
representation of the scrollbar.
Tests have been added to test the scrollbar rendering and placement
accordingly.
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.
