Fix micro spacing inconsistency in SpaceHierarchyDelegate

Exposing index allows RoundedItemDelegate to use a consistent padding
for the first and last item in the listview.

src/qml/SpaceHierarchyDelegate.qml

@@ -13,6 +13,8 @@ import org.kde.neochat
 
 Item {
     id: root
+
+    required property int index
     required property TreeView treeView
     required property bool isTreeNode
     required property bool expanded