core: layout: mark layout switcher for Greek math as non-text

The "switch_greekmath" key doesn't emit text. The Shift modifier
shouldn't make the key label uppercase. Mark it as a key that doesn't
emit text.

src/core/layout.rs

@@ -283,6 +283,8 @@ impl Part {
             KeyValue(Keysym::Meta_L, _) => false,
             KeyValue(Keysym::XF86_Fn, _) => false,
 
+            KeyValue(Keysym::Scroll_Lock, _) => false,
+
             KeyValue(_, l) => l != "",
         }
     }