Add support for horizontal scroll

[preiter93]

• Make list configureable with a fluent direction setter
• The direction setter should take a Direction enum
• The default should be Direction::Vertical
• Direction::Horizontal should layout a horizontally scrollable list
• Implementation should be straightforward, switching the main-axis size from height to width
• An example demo should be added, preferably showing both a vertical and horizontal list in one demo.

This might be a breaking change, because there is only 1 ListableWidget trait. It should be allowed to use the same widget in a vertical and horizontal list. For this either a new trait must be added which is non breakint, or the current one extended. I would go with the latter, and would extend the main_axis_size method with an input parameter main_axis_direction. With this the user can match on the direction if he wants or omit it.

[preiter93]

Implemented in https://github.com/preiter93/tui-widget-list/pull/12