Loading indicator (#2018)

* loading indicator and tests

* docs

* snapshot

* remove snapshot

* remove debug main [skip ci]

* changelog [skip ci]

* make start time private

docs/examples/widgets/loading_indicator.py

@@ -0,0 +1,12 @@
+from textual.app import App, ComposeResult
+from textual.widgets import LoadingIndicator
+
+
+class LoadingApp(App):
+    def compose(self) -> ComposeResult:
+        yield LoadingIndicator()
+
+
+if __name__ == "__main__":
+    app = LoadingApp()
+    app.run()