Features
* `widgets.Container`: added `add_child`, `del_child_by_name`, and `del_child_by_widget` methods, and updated methods for deleting and adding tabs in `widgets.TabBox` to be consistent.
* `widgets.Box`: added `border` and `border_label` properties.
* `widgets.Scrollable`: switched `scrollable` and `get/set_scroll_direction` methods over to properties.
* `widgets.List`: switched `active_child` method over to property
Bugfixes
* Fix crash on user input in 0.0.1 due to wrong parsy version.