GUAC-1213: Add date/time field format placeholders.

guacamole/src/main/webapp/app/form/templates/dateField.html

@@ -2,6 +2,7 @@
     <input type="date"
            ng-model="typedValue"
            ng-model-options="modelOptions"
+           placeholder="{{'FORM.FIELD_PLACEHOLDER_DATE' | translate}}"
            autocorrect="off"
            autocapitalize="off"/>
 </div>

guacamole/src/main/webapp/app/form/templates/timeField.html

@@ -2,6 +2,7 @@
     <input type="time"
            ng-model="typedValue"
            ng-model-options="modelOptions"
+           placeholder="{{'FORM.FIELD_PLACEHOLDER_TIME' | translate}}"
            autocorrect="off"
            autocapitalize="off"/>
 </div>