Implement a Progress Bar widget. (#2333)

* First prototype of PB.

* Repurpose UnderlineBar.

* Factor out 'Bar' widget.

* Revert "Factor out 'Bar' widget."

This reverts commit 0bb4871adf.

* Add Bar widget.

* Cap progress at 100%.

* Add skeleton for the ETA label.

[skip ci]

* Add ETA display.

* Improve docstrings.

* Directly compute percentage.

* Watch percentage changes directly.

[skip ci]

* Documentation.

* Make reactive percentage private.

Instead, we create a public read-only percentage property.

* Update griffe to fix documentation issue.

Related issues: #1572, https://github.com/mkdocstrings/griffe/issues/128.
Related PRs: https://github.com/mkdocstrings/griffe/pull/135.

* Add example and docs.

* Address review feedback.

[skip ci]

* More documentation.

* Add tests.

* Changelog.

* More tests.

* Fix/fake tests.

* Final tweaks.

docs/examples/widgets/progress_bar_isolated_.py

@@ -0,0 +1,46 @@
+from textual.app import App, ComposeResult
+from textual.containers import Center, Middle
+from textual.timer import Timer
+from textual.widgets import Footer, ProgressBar
+
+
+class IndeterminateProgressBar(App[None]):
+    BINDINGS = [("s", "start", "Start")]
+
+    progress_timer: Timer
+    """Timer to simulate progress happening."""
+
+    def compose(self) -> ComposeResult:
+        with Center():
+            with Middle():
+                yield ProgressBar()
+        yield Footer()
+
+    def on_mount(self) -> None:
+        """Set up a timer to simulate progess happening."""
+        self.progress_timer = self.set_interval(1 / 10, self.make_progress, pause=True)
+
+    def make_progress(self) -> None:
+        """Called automatically to advance the progress bar."""
+        self.query_one(ProgressBar).advance(1)
+
+    def action_start(self) -> None:
+        """Start the progress tracking."""
+        self.query_one(ProgressBar).update(total=100)
+        self.progress_timer.resume()
+
+    def key_f(self) -> None:
+        # Freeze time for the indeterminate progress bar.
+        self.query_one(ProgressBar).query_one("#bar")._get_elapsed_time = lambda: 5
+
+    def key_t(self) -> None:
+        # Freeze time to show always the same ETA.
+        self.query_one(ProgressBar).query_one("#eta")._get_elapsed_time = lambda: 3.9
+        self.query_one(ProgressBar).update(total=100, progress=39)
+
+    def key_u(self) -> None:
+        self.query_one(ProgressBar).update(total=100, progress=100)
+
+
+if __name__ == "__main__":
+    IndeterminateProgressBar().run()