Add vendored TreeView components
This MR includes vendored versions of the Kirigami Addons TreeListView, InternalTreeListView, and BasicTreeItem, which are used in the tasks view.
This should be removed once Kirigami Addons passes KDE Review.
This MR includes vendored versions of the Kirigami Addons TreeListView, InternalTreeListView, and BasicTreeItem, which are used in the tasks view.
This should be removed once Kirigami Addons passes KDE Review.