Make it clear, in the code, that all containers are considered scrollable

The code is exactly the same between is_container and is_scrollable, and the
intent (confirmed in
Textualize#1343 (comment))
is that the latter is intended to be overridden in some circumstances (so
far only in `ScrollView`). As such I feel it better conveys intent and
reduces the changes of mismatched future changes if is_scrollable is defined
in respect to is_container.

The only possible reason I can think of is if there's a measurable
performance win here. Applying Knuth for the moment, at least for the scope
of this PR. I strongly suspect this is one of the 97% rather than one of the
3% and for the purposes of moving stuff around (which I may be doing as I
explore this) I believe this makes it easier to follow and to think about.

Author: davep

src/textual/widget.py

@@ -1227,7 +1227,7 @@ def is_scrollable(self) -> bool:
         Returns:
             bool: True if this widget may be scrolled.
         """
-        return self.styles.layout is not None or bool(self.children)
+        return self.is_container
 
     @property
     def layer(self) -> str: