Add an example of using a RadioSet.Changed message (#1935)

Unlike a few other widgets, the RadioSet is pretty much all about reacting
to the selection result; the question of how you go about it has already
come up and while the message is documented, complete with all properties,
it can't hurt to have an illustrative example of code that uses it.

Here I add an extra RadioSet example that sits with the message in the
reference. This should help the reader better follow how to use it, and also
gives something to link to if someone hasn't got that far into the
documentation yet but is attempting to use the RadioSet.

docs/examples/widgets/radio_set_changed.css

@@ -0,0 +1,12 @@
+Vertical {
+    align: center middle;
+}
+
+Horizontal {
+    align: center middle;
+    height: auto;
+}
+
+RadioSet {
+    width: 45%;
+}