[working] add grid filter

lib/ui/grid/grid.html

@@ -198,6 +198,7 @@
           key: 'c2a',
           caption: '下拉',
           type: Grid.ColumnTypes.Dropdown,
+          allowFilter: true,
           source: item => {
             if (item.source == null) {
               return new Promise((resolve, reject) => {