Add support for sending a message when the selection list changes

The developer using this may wish to react to UI changes being made, but
they may also want to just know when the collection of selected values has
changed -- this could happen via code so won't get any UI/IO messages. So
this adds a message that is always sent when a change to the collection of
selected values happens.

src/textual/widgets/_selection_list.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from dataclasses import dataclass
 from typing import ClassVar, Generic, TypeVar, cast
 
 from rich.repr import Result
@@ -162,6 +163,18 @@ class SelectionList(Generic[SelectionType], OptionList):
         `SelectionList` or in a parent node in the DOM.
         """
 
+    @dataclass
+    class SelectedChanged(Generic[MessageSelectionType], Message):
+        """Message sent when the collection of selected values changes."""
+
+        selection_list: SelectionList[MessageSelectionType]
+        """The `SelectionList` that sent the message."""
+
+        @property
+        def control(self) -> SelectionList[MessageSelectionType]:
+            """An alias for `selection_list`."""
+            return self.selection_list
+
     def __init__(
         self,
         *selections: tuple[TextType, SelectionType]
@@ -200,7 +213,9 @@ class SelectionList(Generic[SelectionType], OptionList):
         Args:
             value: The value to mark as selected.
         """
+        if value not in self._selected:
             self._selected[value] = None
+            self.post_message(self.SelectedChanged(self))
 
     def select(self, selection: Selection[SelectionType] | SelectionType) -> Self:
         """Mark the given selection as selected.
@@ -224,6 +239,7 @@ class SelectionList(Generic[SelectionType], OptionList):
         """
         try:
             del self._selected[value]
+            self.post_message(self.SelectedChanged(self))
         except KeyError:
             pass