Checkbox polishing + fix auto-width in Horizontal layout (#942)

* checkbox widget

* fixes

* Checkbox additions, fix content width in horizontal layout

* Update docs, add tests for checkbox

* Remove some test code

* Small renaming of test class

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

docs/widgets/checkbox.md

@@ -0,0 +1,57 @@
+# Checkbox
+
+A simple checkbox widget which stores a boolean value.
+
+- [x] Focusable
+- [ ] Container
+
+## Example
+
+The example below shows checkboxes in various states.
+
+=== "Output"
+
+    ```{.textual path="docs/examples/widgets/checkbox.py"}
+    ```
+
+=== "checkbox.py"
+
+    ```python
+    --8<-- "docs/examples/widgets/checkbox.py"
+    ```
+
+=== "checkbox.css"
+
+    ```css
+    --8<-- "docs/examples/widgets/checkbox.css"
+    ```
+
+## Reactive Attributes
+
+| Name    | Type   | Default | Description                        |
+|---------|--------|---------|------------------------------------|
+| `value` | `bool` | `False` | The default value of the checkbox. |
+
+## Messages
+
+### Pressed
+
+The `Checkbox.Changed` message is sent when the checkbox is toggled.
+
+- [x] Bubbles
+
+#### Attributes
+
+| attribute | type   | purpose                        |
+|-----------|--------|--------------------------------|
+| `value`   | `bool` | The new value of the checkbox. |
+
+## Additional Notes
+
+- To remove the spacing around a checkbox, set `border: none;` and `padding: 0;`.
+- The `.checkbox--switch` component class can be used to change the color and background of the switch.
+- When focused, the ++enter++ or ++space++ keys can be used to toggle the checkbox.
+
+## See Also
+
+- [Checkbox](../reference/checkbox.md) code reference