Rename Checkbox to Switch

A new form of Checkbox will be arriving in Textual soon, working in
conjunction with a RadioButton. What was called Checkbox is perhaps a wee
bit heavyweight in terms of visual design, but is a style of widget that
should remain.

With this in mind we're renaming the current Checkbox to Switch. In all
other respects its workings remains the same, only the name has changed.

Things for people to watch out for:

- Imports will need to be updated.
- Queries will need to be updated; special attention will need to be paid to
  any queries that are string-based.
- CSS will need to be changed if any Checkbox styling is happening, or if
  any Checkbox component styles are being used.

See #1725 as the initial motivation and #1746 as the issue for this
particular change.

docs/widgets/checkbox.md

@@ -1,63 +0,0 @@
-# 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"
-
-    ```sass
-    --8<-- "docs/examples/widgets/checkbox.css"
-    ```
-
-## Reactive Attributes
-
-| Name    | Type   | Default | Description                        |
-| ------- | ------ | ------- | ---------------------------------- |
-| `value` | `bool` | `False` | The default value of the checkbox. |
-
-## Bindings
-
-The checkbox widget defines directly the following bindings:
-
-::: textual.widgets.Checkbox.BINDINGS
-    options:
-      show_root_heading: false
-      show_root_toc_entry: false
-
-## Component Classes
-
-The checkbox widget provides the following component classes:
-
-::: textual.widgets.Checkbox.COMPONENT_CLASSES
-    options:
-      show_root_heading: false
-      show_root_toc_entry: false
-
-## Messages
-
-### ::: textual.widgets.Checkbox.Changed
-
-## Additional Notes
-
-- To remove the spacing around a checkbox, set `border: none;` and `padding: 0;`.
-
-## See Also
-
-- [Checkbox](../api/checkbox.md) code reference