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/examples/widgets/checkbox.css

@@ -0,0 +1,28 @@
+Screen {
+    align: center middle;
+}
+
+.container {
+    height: auto;
+    width: auto;
+}
+
+Checkbox {
+    height: auto;
+    width: auto;
+}
+
+.label {
+    height: 3;
+    content-align: center middle;
+    width: auto;
+}
+
+#custom-design {
+    background: darkslategrey;
+}
+
+#custom-design > .checkbox--switch {
+    color: dodgerblue;
+    background: darkslateblue;
+}

docs/examples/widgets/checkbox.py

@@ -0,0 +1,33 @@
+from textual.app import App, ComposeResult
+from textual.containers import Horizontal
+from textual.widgets import Checkbox, Static
+
+
+class CheckboxApp(App):
+    def compose(self) -> ComposeResult:
+        yield Static("[b]Example checkboxes\n", classes="label")
+        yield Horizontal(
+            Static("off:     ", classes="label"), Checkbox(), classes="container"
+        )
+        yield Horizontal(
+            Static("on:      ", classes="label"),
+            Checkbox(value=True),
+            classes="container",
+        )
+
+        focused_checkbox = Checkbox()
+        focused_checkbox.focus()
+        yield Horizontal(
+            Static("focused: ", classes="label"), focused_checkbox, classes="container"
+        )
+
+        yield Horizontal(
+            Static("custom:  ", classes="label"),
+            Checkbox(id="custom-design"),
+            classes="container",
+        )
+
+
+app = CheckboxApp(css_path="checkbox.css")
+if __name__ == "__main__":
+    app.run()