Bugfixes on link deletion, and use a GET form

Use a GET form to delete links: harmonize with edit_link and preparation for #585

Bug fixes:

  * LinkDB element can't be passed as reference, fix error:

    PHP Notice:  Indirect modification of overloaded element of LinkDB has no effect

  * Resource cache folder setting wasn't set correctly

tpl/linklist.html

@@ -84,7 +84,7 @@
                             <input type="hidden" name="edit_link" value="{$value.id}">
                             <input type="image" alt="Edit" src="images/edit_icon.png#" title="Edit" class="button_edit">
                         </form><br>
-                        <form method="POST" class="buttoneditform">
+                        <form method="GET" class="buttoneditform">
                             <input type="hidden" name="lf_linkdate" value="{$value.id}">
                             <input type="hidden" name="token" value="{$token}">
                             <input type="hidden" name="delete_link">