Make a start on the SelectionList example apps

docs/examples/widgets/selection_list_selections.py

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+from textual.app import App, ComposeResult
+from textual.widgets import Footer, Header, SelectionList
+from textual.widgets.selection_list import Selection
+
+
+class SelectionListApp(App[None]):
+    CSS_PATH = "selection_list.css"
+
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield SelectionList[int](
+            Selection("Falken's Maze", 0, True),
+            Selection("Black Jack", 1),
+            Selection("Gin Rummy", 2),
+            Selection("Hearts", 3),
+            Selection("Bridge", 4),
+            Selection("Checkers", 5),
+            Selection("Chess", 6, True),
+            Selection("Poker", 7),
+            Selection("Fighter Combat", 8, True),
+        )
+        yield Footer()
+
+    def on_mount(self) -> None:
+        self.query_one(SelectionList).border_title = "Shall we play some games?"
+
+
+if __name__ == "__main__":
+    SelectionListApp().run()