Simplify ‚Add type‘ popup in folder view.

- Removed Label (really, just made it invisible, so it is only visible to screen readers)
- Added Disabled initial label, that acts as a default selection so the popup explains itself
- Ensured that after a selection the popup is reset to the default choice.

All these changes achieve a much simpler add menu, that reliably works even if you want to select the same entry multiple times without reloading the page (i.e. when navigating back). 

Before these changes, this required the add button, as you could otherwise not add the same kind of element twice.

src/OFS/zpt/main.zpt

@@ -8,19 +8,19 @@
       tal:attributes="action string:${request/URL1}/" >
   <div class="form-group">
     <label for="addItemSelect"
-           class="mr-2">Select type to add</label>
+           class="mr-2 sr-only">Select type to add</label>
     <select class="form-control mr-2"
             name=":action"
             id="addItemSelect"
-            tal:attributes="onchange string:location.href='${request/URL1}/'+this.options[this.selectedIndex].value">
+            tal:attributes="onchange string:location.href='${request/URL1}/'+this.options[this.selectedIndex].value;;this.selectedIndex = 0">
+      <option value='' disabled selected>Select type to add</option>
       <tal:option tal:repeat="mt here/filtered_meta_types">
         <option tal:condition="mt/action"
                 tal:attributes="value mt/action"
                 tal:content="mt/name">Action</option>
       </tal:option>
     </select>
   </div>
-  <input class="btn btn-outline-secondary" type="submit" name="submit" value="Add" />
 </form>
 
 <form name="objectItems"