implemented unlinking task from parent task

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

frontend/src/routes/task/View.svelte

@@ -140,6 +140,10 @@
         showSettings = !showSettings;
     }
 
+    function unlink_parent(){
+        update({parent_task_id:null});
+    }
+
     async function update(data){
         const url  = api(`task/${id}`);
         const resp = await fetch(url,{
@@ -151,7 +155,11 @@
             yikes();
             let old_task = task;
             task  = await resp.json();
-            if (task.parent_id == old_task.parent_id) task.parent = old_task.parent;
+            if (!task.parent_id){
+                task.parent = null;
+            } else {
+                if (task.parent_id == old_task.parent_id) task.parent = old_task.parent;
+            }
             return true;
         } else {
             error(resp);
@@ -196,6 +204,7 @@
     <div>{t('parent_task')}</div>
     <div class="parent">
         <a href="#" onclick={gotoParent}>{task.parent.name}</a>
+        <button class="symbol" title={t('unlink')} onclick={unlink_parent}></button>
     </div>
     {/if}
     <div>{t('task')}</div>