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_styled.css

@@ -0,0 +1,22 @@
+Bar > .bar--indeterminate {
+    color: $primary;
+    background: $secondary;
+}
+
+Bar > .bar--bar {
+    color: $primary;
+    background: $primary 30%;
+}
+
+Bar > .bar--complete {
+    color: $error;
+}
+
+PercentageStatus {
+    text-style: reverse;
+    color: $secondary;
+}
+
+ETAStatus {
+    text-style: underline;
+}