Todo: Store icons in the settings via index instead of strings

This allows us to use something else than a string in order to reference
icons. For an upcoming patch this will be necessary.

Since this patch introduces a settings structure change, a migration
feature from the old "Keyword\iconResource" string to the new "Keyword
\iconType" int is implemented.

Change-Id: Ia5695418fb135510ed549cf9a7cb59aab5389f31
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>

src/plugins/todo/todoitemsmodel.cpp

@@ -100,7 +100,7 @@ QVariant TodoItemsModel::data(const QModelIndex &index, int role) const
                 case Qt::DisplayRole:
                     return item.text;
                 case Qt::DecorationRole:
-                    return QVariant::fromValue(Utils::ThemeHelper::themedIcon(item.iconResource));
+                    return icon(item.iconType);
             }
             break;