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>

tests/layouts/test_horizontal.py

@@ -0,0 +1,46 @@
+from textual.geometry import Size
+from textual.layouts.horizontal import HorizontalLayout
+from textual.widget import Widget
+
+
+class SizedWidget(Widget):
+    """Simple Widget wrapped allowing you to modify the return values for
+    get_content_width and get_content_height via the constructor."""
+
+    def __init__(
+        self,
+        *children: Widget,
+        content_width: int = 10,
+        content_height: int = 5,
+    ):
+        super().__init__(*children)
+        self.content_width = content_width
+        self.content_height = content_height
+
+    def get_content_width(self, container: Size, viewport: Size) -> int:
+        return self.content_width
+
+    def get_content_height(self, container: Size, viewport: Size, width: int) -> int:
+        return self.content_height
+
+
+CHILDREN = [
+    SizedWidget(content_width=10, content_height=5),
+    SizedWidget(content_width=4, content_height=2),
+    SizedWidget(content_width=12, content_height=3),
+]
+
+
+def test_horizontal_get_content_width():
+    parent = Widget(*CHILDREN)
+    layout = HorizontalLayout()
+    width = layout.get_content_width(widget=parent, container=Size(), viewport=Size())
+    assert width == sum(child.content_width for child in CHILDREN)
+
+
+def test_horizontal_get_content_width_no_children():
+    parent = Widget()
+    layout = HorizontalLayout()
+    container_size = Size(24, 24)
+    width = layout.get_content_width(widget=parent, container=container_size, viewport=Size())
+    assert width == container_size.width