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.py

@@ -0,0 +1,40 @@
+from textual.app import App, ComposeResult
+from textual.containers import Center, VerticalScroll
+from textual.widgets import Button, Header, Input, Label, ProgressBar
+
+
+class FundingProgressApp(App[None]):
+    CSS_PATH = "progress_bar.css"
+
+    TITLE = "Funding tracking"
+
+    def compose(self) -> ComposeResult:
+        yield Header()
+        with Center():
+            yield Label("Funding: ")
+            yield ProgressBar(total=100, show_eta=False)  # (1)!
+        with Center():
+            yield Input(placeholder="$$$")
+            yield Button("Donate")
+
+        yield VerticalScroll(id="history")
+
+    def on_button_pressed(self) -> None:
+        self.add_donation()
+
+    def on_input_submitted(self) -> None:
+        self.add_donation()
+
+    def add_donation(self) -> None:
+        text_value = self.query_one(Input).value
+        try:
+            value = int(text_value)
+        except ValueError:
+            return
+        self.query_one(ProgressBar).advance(value)
+        self.query_one(VerticalScroll).mount(Label(f"Donation for ${value} received!"))
+        self.query_one(Input).value = ""
+
+
+if __name__ == "__main__":
+    FundingProgressApp().run()