Add support for deeply expanding/collapsing/toggling nodes

This commit moves the bulk of the work of each action into an internal
method that does everything *apart* from invalidating the tree. The idea
being that all of the expanded states get updated, all of the update counts
get updated, and then finally one single tree invalidation takes place (the
latter taking place in the public method, which calls the related internal
method).

See Textualize#1430.

src/textual/widgets/_tree.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import ClassVar, Generic, NewType, TypeVar
+from typing import ClassVar, Generic, Iterator, NewType, TypeVar
 
 import rich.repr
 from rich.style import NULL_STYLE, Style
@@ -159,22 +159,67 @@ def allow_expand(self, allow_expand: bool) -> None:
         self._allow_expand = allow_expand
         self._updates += 1
 
-    def expand(self) -> None:
-        """Expand a node (show its children)."""
+    def _expand(self, expand_all) -> None:
+        """Mark a node as expanded (its children are shown).
+
+        Args:
+            expand_all: If `True` expand the all offspring at all depths.
+        """
         self._expanded = True
         self._updates += 1
+        if expand_all:
+            for child in self.children:
+                child._expand(expand_all=True)
+
+    def expand(self, expand_all: bool = False) -> None:
+        """Expand a node (show its children).
+
+        Args:
+            expand_all: If `True` expand the all offspring at all depths.
+        """
+        self._expand(expand_all)
         self._tree._invalidate()
 
-    def collapse(self) -> None:
-        """Collapse the node (hide children)."""
+    def _collapse(self, collapse_all: bool) -> None:
+        """Mark a node as collapsed (its children are hidden).
+
+        Args:
+            collapse_all: If `True` collapse the all offspring at all depths.
+        """
         self._expanded = False
+        if collapse_all:
+            for child in self.children:
+                child._collapse(collapse_all=True)
         self._updates += 1
+
+    def collapse(self, collapse_all: bool = True) -> None:
+        """Collapse the node (hide children).
+
+        Args:
+            collapse_all: If `True` collapse the all offspring at all depths.
+        """
+        self._collapse(collapse_all)
         self._tree._invalidate()
 
-    def toggle(self) -> None:
-        """Toggle the expanded state."""
+    def _toggle(self, toggle_all: bool) -> None:
+        """Toggle the expanded state of the node.
+
+        Args:
+            toggle_all: If `True` toggle the all offspring at all depths.
+        """
         self._expanded = not self._expanded
+        if toggle_all:
+            for child in self.children:
+                child._toggle(toggle_all=True)
         self._updates += 1
+
+    def toggle(self, toggle_all: bool = True) -> None:
+        """Toggle the expanded state.
+
+        Args:
+            toggle_all: If `True` toggle the all offspring at all depths.
+        """
+        self._toggle(toggle_all)
         self._tree._invalidate()
 
     @property