Change X in the ManageServer Activity by - to delete servers

Fixes #25

app/src/main/res/layout/server_item.xml

@@ -5,7 +5,7 @@
     <ImageView
         android:layout_alignParentRight="true"
         android:layout_alignParentTop="true"
-        android:src="@android:drawable/ic_delete"
+        android:src="@drawable/ic_action_remove"
         android:id="@+id/server_delete"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content" />