Move disabled out of Button and into Widget

This doesn't go close to what #1748 is intending to do, but moves `disabled`
to where I want it and keeps `Button` working as before.

src/textual/widget.py

@@ -225,6 +225,8 @@ class Widget(DOMNode):
     """Rich renderable may shrink."""
     auto_links = Reactive(True)
     """Widget will highlight links automatically."""
+    disabled = Reactive(False)
+    """The disabled state of the widget. `True` if disabled, `False if not."""
 
     hover_style: Reactive[Style] = Reactive(Style, repaint=False)
     highlight_link_id: Reactive[str] = Reactive("")

src/textual/widgets/_button.py

@@ -156,9 +156,6 @@ class Button(Static, can_focus=True):
     variant = reactive("default")
     """The variant name for the button."""
 
-    disabled = reactive(False)
-    """The disabled state of the button; `True` if disabled, `False` if not."""
-
     class Pressed(Message, bubble=True):
         """Event sent when a `Button` is pressed.