Help: Use nullptr and fix a few warnings

Change-Id: I7f7307a765d8755bf96b0cad1210e61df0a0199f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/help/localhelpmanager.h

@@ -57,7 +57,7 @@ public:
         ShowLastPages = 2,
     };
 
-    LocalHelpManager(QObject *parent = 0);
+    LocalHelpManager(QObject *parent = nullptr);
     ~LocalHelpManager();
 
     static LocalHelpManager *instance();