GUAC-1193: Add filter placeholder to search input field.

guacamole/src/main/webapp/app/settings/templates/settingsConnectionHistory.html

@@ -26,7 +26,7 @@
 
     <!-- Search controls -->
     <div class="filter">
-        <input class="search-string" type="text" ng-model="searchString" />
+        <input class="search-string" type="text" placeholder="{{'SETTINGS_CONNECTION_HISTORY.FIELD_PLACEHOLDER_FILTER' | translate}}" ng-model="searchString" />
         <button class="search-button" ng-click="search()">{{'SETTINGS_CONNECTION_HISTORY.ACTION_SEARCH' | translate}}</button> 
     </div>