Clean query string when switching pages

If we load a page without a query string, such as Projects->List,
My->Changes, or Settings, it might be confusing to show the old query
string from the previous page.  The query string is now cleared out
when switching pages, leaving the help text visible.

Change-Id: Id4f211d4f874b1acd0c4d6533eee9984b42cc41e

gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/Screen.java

@@ -51,6 +51,7 @@ public abstract class Screen extends View {
     if (header == null) {
       onInitUI();
     }
+    Gerrit.setQueryString(null);
   }
 
   public void registerKeys() {