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.
Improve some examples to provide visual feedback, i.e. for the button
exmample, with fixes to make them compilable with the `Scrollable`
element changes.
This also contains some minor refactoring to improve the readability
and understandability of the library (i.e. renaming of Style.Attributes
to Style.Emphasis).
