Tree view components
add components for displaying tree models wit hthe usual tree representation with expanders. Components are:
- TreeViewDecoration: just the expanders, which can be put in any delegate of a view which is displaying a KDescendantsProxyModel
- AbstractTreeItem: a Kirigami AbstractListItem with an expander inside
- BasicTreeItem: a Kirigami BasicListItem with an expander inside
- TreeListView a ListView which already loads a KDescendantsProxyModel and has tree expand/collapse animations, similar to the QWidget one
- TreeTableView a TableView which already loads a KDescendantsProxyModel