Start the SelectionList messages

It would be nice to just inherit form the OptionList messages, but the
naming of the properties wouldn't quite make sense, and there's also the
generic typing issue too. So here I start to spin up my own messages down
here.

Also, as an initial use of this, grab the OptionList highlight message and
turn it onto one of out own.

src/textual/widgets/_selection_list.py

@@ -2,19 +2,23 @@
 
 from __future__ import annotations
 
-from typing import ClassVar, Generic, TypeVar
+from typing import ClassVar, Generic, TypeVar, cast
 
+from rich.repr import Result
 from rich.segment import Segment
 from rich.style import Style
 from rich.text import TextType
 
 from ..binding import Binding
+from ..messages import Message
 from ..strip import Strip
 from ._option_list import Option, OptionList
 from ._toggle_button import ToggleButton
 
 SelectionType = TypeVar("SelectionType")
 """The type for the value of a `Selection`"""
+MessageSelectionType = TypeVar("MessageSelectionType")
+"""The type for the value of a `SelectionList` message"""
 
 
 class Selection(Generic[SelectionType], Option):
@@ -102,6 +106,48 @@ class SelectionList(Generic[SelectionType], OptionList):
     }
     """
 
+    class SelectionMessage(Generic[MessageSelectionType], Message):
+        """Base class for all selection messages."""
+
+        def __init__(self, selection_list: SelectionList, index: int) -> None:
+            """Initialise the selection message.
+
+            Args:
+                selection_list: The selection list that owns the selection.
+                index: The index of the selection that the message relates to.
+            """
+            super().__init__()
+            self.selection_list: SelectionList = selection_list
+            """The option list that sent the message."""
+            self.selection: Selection[MessageSelectionType] = cast(
+                Selection[MessageSelectionType],
+                selection_list.get_option_at_index(index),
+            )
+            """The highlighted selection."""
+            self.selection_index: int = index
+            """The index of the selection that the message relates to."""
+
+        @property
+        def control(self) -> OptionList:
+            """The option list that sent the message.
+
+            This is an alias for [`OptionMessage.option_list`][textual.widgets.OptionList.OptionMessage.option_list]
+            and is used by the [`on`][textual.on] decorator.
+            """
+            return self.selection_list
+
+        def __rich_repr__(self) -> Result:
+            yield "selection_list", self.selection_list
+            yield "selection", self.selection
+            yield "selection_index", self.selection_index
+
+    class SelectionHighlighted(SelectionMessage):
+        """Message sent when a selection is highlighted.
+
+        Can be handled using `on_selection_list_selection_highlighted` in a subclass of
+        `SelectionList` or in a parent node in the DOM.
+        """
+
     def __init__(
         self,
         *selections: tuple[TextType, SelectionType]
@@ -228,3 +274,14 @@ class SelectionList(Generic[SelectionType], OptionList):
                 *prompt,
             ]
         )
+
+    def _on_option_list_option_highlighted(
+        self, event: OptionList.OptionHighlighted
+    ) -> None:
+        """Capture the `OptionList` highlight event and turn it into a `SelectionList` event.
+
+        Args:
+            event: The event to capture and recreate.
+        """
+        event.stop()
+        self.post_message(self.SelectionHighlighted(self, event.option_index))