window-list: Don't rely solely on -st-natural-width

-st-natural-width is useful to request a larger-than-usual width,
add back the max-width removed in commit 702cf52cfc to also request
a smaller-than-usual width as necessary.

extensions/window-list/stylesheet.css

@@ -24,6 +24,7 @@
 
 .window-button > StWidget {
   -st-natural-width: 250px;
+  max-width: 250px;
   color: #bbb;
   background-color: black;
   border-radius: 4px;