* Add get_child_by_id and get_widget_by_id
* Remove redundant code
* Add unit tests for app-level get_child_by_id and get_widget_by_id
* Remove redundant test fixture injection
* Update CHANGELOG
* Enforce uniqueness of ID amongst widget children
* Enforce unique widget IDs amongst widgets mounted together
* Update CHANGELOG.md
* Ensuring unique IDs in a more logical place
* Add docstring to NodeList._get_by_id
* Dont use duplicate IDs in tests, dont mount 2000 widgets
* Mounting less widgets in a unit test
* Reword error message
* Use lower-level depth first search in get_widget_by_id to break out early
Because some changes are going to be made to the way that removal of widgets
happens, and because this will affect the linkage between parents and
children, and because we don't want the current way of working to change...
extend to the tests to be sure that when an unmount happens a widget has no
children any more, but still knows about its parent.
