Add a focusable property

We want to maintain `can_focus` as "this is a thing that can, at some point,
receive focus, assuming it isn't disabled". So `can_focus` is now very much
about the ability to receive focus at all.

The new `focusable` property becomes about "can this widget receive focus
right now?".

[This is a rewording of a previous commit -- I get the wrong thing]

src/textual/widget.py

@@ -1177,6 +1177,11 @@ class Widget(DOMNode):
         """
         return self.virtual_region.grow(self.styles.margin)
 
+    @property
+    def focusable(self) -> bool:
+        """Can this widget currently receive focus?"""
+        return self.can_focus and not self.disabled
+
     @property
     def focusable_children(self) -> list[Widget]:
         """Get the children which may be focused.