Add enabled and disabled pseudo-classes

Note that this doesn't touch the application of stylesheets yet, in terms of
things like specificity; this just makes sure that the classes exist and can
be seen.

docs/guide/CSS.md

@@ -315,6 +315,8 @@ The `background: green` is only applied to the Button underneath the mouse curso
 
 Here are some other pseudo classes:
 
+- `:disabled` Matches widgets which are in a disabled state.
+- `:enabled` Matches widgets which are in an enabled state.
 - `:focus` Matches widgets which have input focus.
 - `:focus-within` Matches widgets with a focused a child widget.
 

src/textual/widget.py

@@ -2084,6 +2084,7 @@ class Widget(DOMNode):
             Names of the pseudo classes.
 
         """
+        yield "disabled" if self.disabled else "enabled"
         if self.mouse_over:
             yield "hover"
         if self.has_focus: