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>

sandbox/darren/check.py

@@ -0,0 +1,24 @@
+from textual.app import App, ComposeResult
+
+from textual.containers import Container
+from textual.widgets import Checkbox, Footer
+
+
+class CheckboxApp(App):
+    BINDINGS = [("s", "switch", "Press switch"), ("d", "toggle_dark", "Dark mode")]
+
+    def compose(self) -> ComposeResult:
+        yield Footer()
+        yield Container(Checkbox(id="check", animate=True))
+
+    def action_switch(self) -> None:
+        checkbox = self.query_one(Checkbox)
+        checkbox.toggle()
+
+    def key_f(self):
+        print(self.app.focused)
+
+
+app = CheckboxApp(css_path="check.css")
+if __name__ == "__main__":
+    app.run()