Move scroll navigation key bindings out of widget and into own container

The idea here is that not every widget will scroll, and as such not every
widget needs to have default bindings for calling the scrolling methods.
Generally scrolling is something done in a container.

These days we have *Scroll containers. As such it makes sense to introduce
the bindings in a common parent class for those containers.

This commit moves the binding from widget and creates that common parent
class, and then has HorizontalScroll and VerticalScroll inherit from it.

This is, it should be noted, a breaking change. Any code that creates a
scrolling widget that assumes that the bindings are just there, where that
widget doesn't inherit either from HorizontalScroll or VerticalScroll, will
suddenly find that scrolling with the keyboard is no longer possible.

See Textualize#2332.

src/textual/containers.py

@@ -3,7 +3,11 @@
 
 """
 
+from __future__ import annotations
 
+from typing import ClassVar
+
+from .binding import Binding, BindingType
 from .widget import Widget
 
 
@@ -19,6 +23,34 @@ class Container(Widget):
     """
 
 
+class ScrollableContainer(Widget):
+    """Base container widget that binds navigation keys for scrolling."""
+
+    BINDINGS: ClassVar[list[BindingType]] = [
+        Binding("up", "scroll_up", "Scroll Up", show=False),
+        Binding("down", "scroll_down", "Scroll Down", show=False),
+        Binding("left", "scroll_left", "Scroll Up", show=False),
+        Binding("right", "scroll_right", "Scroll Right", show=False),
+        Binding("home", "scroll_home", "Scroll Home", show=False),
+        Binding("end", "scroll_end", "Scroll End", show=False),
+        Binding("pageup", "page_up", "Page Up", show=False),
+        Binding("pagedown", "page_down", "Page Down", show=False),
+    ]
+    """Keyboard bindings for scrollable containers.
+
+    | Key(s) | Description |
+    | :- | :- |
+    | up | Scroll up, if vertical scrolling is available. |
+    | down | Scroll down, if vertical scrolling is available. |
+    | left | Scroll left, if horizontal scrolling is available. |
+    | right | Scroll right, if horizontal scrolling is available. |
+    | home | Scroll to the home position, if scrolling is available. |
+    | end | Scroll to the end position, if scrolling is available. |
+    | pageup | Scroll up one page, if vertical scrolling is available. |
+    | pagedown | Scroll down one page, if vertical scrolling is available. |
+    """
+
+
 class Vertical(Widget):
     """A container which arranges children vertically."""
 
@@ -31,7 +63,7 @@ class Vertical(Widget):
     """
 
 
-class VerticalScroll(Widget, can_focus=True):
+class VerticalScroll(ScrollableContainer, can_focus=True):
     """A container which arranges children vertically, with an automatic vertical scrollbar."""
 
     DEFAULT_CSS = """
@@ -55,7 +87,7 @@ class Horizontal(Widget):
     """
 
 
-class HorizontalScroll(Widget, can_focus=True):
+class HorizontalScroll(ScrollableContainer, can_focus=True):
     """A container which arranges children horizontally, with an automatic horizontal scrollbar."""
 
     DEFAULT_CSS = """

src/textual/widget.py

@@ -55,7 +55,6 @@
 from ._styles_cache import StylesCache
 from .actions import SkipAction
 from .await_remove import AwaitRemove
-from .binding import Binding
 from .box_model import BoxModel
 from .css.query import NoMatches, WrongType
 from .css.scalar import ScalarOffset
@@ -238,17 +237,6 @@ class Widget(DOMNode):
 
     """
 
-    BINDINGS = [
-        Binding("up", "scroll_up", "Scroll Up", show=False),
-        Binding("down", "scroll_down", "Scroll Down", show=False),
-        Binding("left", "scroll_left", "Scroll Up", show=False),
-        Binding("right", "scroll_right", "Scroll Right", show=False),
-        Binding("home", "scroll_home", "Scroll Home", show=False),
-        Binding("end", "scroll_end", "Scroll End", show=False),
-        Binding("pageup", "page_up", "Page Up", show=False),
-        Binding("pagedown", "page_down", "Page Down", show=False),
-    ]
-
     DEFAULT_CSS = """
     Widget{
         scrollbar-background: $panel-darken-1;