Add get_child_by_id and get_widget_by_id (#1146)

* 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

tests/test_widget_removing.py

@@ -15,18 +15,18 @@ async def test_remove_single_widget():
 async def test_many_remove_all_widgets():
     """It should be possible to remove all widgets on a multi-widget screen."""
     async with App().run_test() as pilot:
-        await pilot.app.mount(*[Static() for _ in range(1000)])
-        assert len(pilot.app.screen.children) == 1000
+        await pilot.app.mount(*[Static() for _ in range(10)])
+        assert len(pilot.app.screen.children) == 10
         await pilot.app.query(Static).remove()
         assert len(pilot.app.screen.children) == 0
 
 async def test_many_remove_some_widgets():
     """It should be possible to remove some widgets on a multi-widget screen."""
     async with App().run_test() as pilot:
-        await pilot.app.mount(*[Static(id=f"is-{n%2}") for n in range(1000)])
-        assert len(pilot.app.screen.children) == 1000
-        await pilot.app.query("#is-0").remove()
-        assert len(pilot.app.screen.children) == 500
+        await pilot.app.mount(*[Static(classes=f"is-{n%2}") for n in range(10)])
+        assert len(pilot.app.screen.children) == 10
+        await pilot.app.query(".is-0").remove()
+        assert len(pilot.app.screen.children) == 5
 
 async def test_remove_branch():
     """It should be possible to remove a whole branch in the DOM."""