Validation (#2600)

* A few different types of validation

* Rename

* Fix test

* Updating validation framework

* Update lockfile

* Ensure validators can be None

* Reworking the API a little

* Convert Input.Changed to dataclass

* Add utility for getting failures as strings

* Update an example in Validator docstring

* Remove some redundant `pass`es

* Renaming variables

* Validating Input on submit, attaching result to Submitted event

* Testing various validation features

* Update snapshots and deps

* Styling unfocused -invalid Input differently

* Add snapshot test around input validation and associated styles

* Validation docs

* Tidying validation docs in Input widget reference

* Fix mypy issues

* Remove __bool__ from Failure, make validator field required

* Code review changes

* Improving error messages in Validators

tests/test_data_table.py

@@ -1010,6 +1010,7 @@ async def test_scrolling_cursor_into_view():
         table.add_rows([(n,) for n in range(300)])
 
         await pilot.press("c")
+        await pilot.pause()
         assert table.scroll_y > 100