window-list: Fill max-width if possible

Items in the window list should take up a fixed amount of space
unless the list is full and items need to shrink. To achieve this,
replace the max-width with the newly added -st-natural-width.

https://bugzilla.gnome.org/show_bug.cgi?id=695392

extensions/window-list/stylesheet.css

@@ -23,7 +23,7 @@
 }
 
 .window-button > StWidget {
-  max-width: 250px;
+  -st-natural-width: 250px;
   color: #bbb;
   background-color: black;
   border-radius: 4px;