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;
+}