Widget collapsible (#2989)

* Collapsible container widget.

* Expose collapsible widget.

* Add collapsible container example

* Rename member variables as label and apply formatting

* Apply hover effect

* Apply formatting

* Add collapsible construction example with children.

* Wrap contents within Container and move _collapsed flag to Collapsible class from  Summary for easier access.

* Add collapsible example that is expanded by default.

* Update collapsed property to be reactive

* Add footer to collapse and expand all with bound keys.

* Expose summary property of Collapsible

* Assign ids of ollapsed, expanded label instead of classes

* Add unit tests of Collapsible

* Rename class Summary to Title

* Rename variables of expanded/collapsed symbols and add it to arguments..

* Add documentation for Collapsible

* Update symbol ids of Collapsible title

* Update src/textual/widgets/_collapsible.py

Correct import path

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

* Sort module names in alphabetical order

* Clarify that collapsible is non-focusable in documentation.

* Add version hint

* Fix documentation of Collapsible.

* Add snapshot test for collapsible widget

* Stop on click event from Collapsible.

* Handle Title.Toggle event to prevent event in Contents from propagating to the children or parents Collapsible widgets.

* Update Collapsible default css to have 1 fraction of width instead of 100%

* Update Collapsible custom symbol snapshot

* Add Collapsible custom symbol snapshot as an example

* Update docs/widgets/collapsible.md

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

* Update src/textual/widgets/_collapsible.py

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

* Fix typo in Collapsible docs

* Rework collapsible documentation.

---------

Co-authored-by: Sunyoung Yoo <luysunyoung@aifactory.page>
Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

docs/examples/widgets/collapsible_custom_symbol.py

@@ -0,0 +1,25 @@
+from textual.app import App, ComposeResult
+from textual.containers import Horizontal
+from textual.widgets import Collapsible, Label
+
+
+class CollapsibleApp(App[None]):
+    def compose(self) -> ComposeResult:
+        with Horizontal():
+            with Collapsible(
+                collapsed_symbol=">>>",
+                expanded_symbol="v",
+            ):
+                yield Label("Hello, world.")
+
+            with Collapsible(
+                collapsed_symbol=">>>",
+                expanded_symbol="v",
+                collapsed=False,
+            ):
+                yield Label("Hello, world.")
+
+
+if __name__ == "__main__":
+    app = CollapsibleApp()
+    app.run()