fix(Filtering): Let mobile devices tap filter box

A preventDefault() in the filter input's ng-click was preventing mobile
users from being able to select the input box. Tapping on it simply did
nothing.

Removing this preventDefault() fixed the issue, and does not seem to have
caused any side effects.

src/templates/ui-grid/uiGridHeaderCell.html

@@ -13,7 +13,7 @@
   </div>
 
   <div ng-if="filterable" class="ui-grid-filter-container" ng-repeat="colFilter in col.filters">
-    <input type="text" class="ui-grid-filter-input" ng-model="colFilter.term" ng-click="$event.stopPropagation()" ng-attr-placeholder="{{colFilter.placeholder || ''}}" />
+    <input type="text" class="ui-grid-filter-input" ng-model="colFilter.term" ng-attr-placeholder="{{colFilter.placeholder || ''}}" />
 
     <div class="ui-grid-filter-button" ng-click="colFilter.term = null">
       <i class="ui-grid-icon-cancel" ng-show="!!colFilter.term">&nbsp;</i> <!-- use !! because angular interprets 'f' as false -->