newsticker: Context menu in treeview's tree.

* net/newst-treeview.el (newsticker--treeview-list-add-item)
(newsticker--treeview-propertize-tag): Bind tree menu to mouse-3.
(newsticker--treeview-create-groups-menu)
(newsticker--treeview-create-tree-menu): Removed.
(newsticker--treeview-tree-open-menu): New.
(newsticker-treeview-tree-click): Pass event to
`newsticker-treeview-tree-do-click'.
(newsticker-treeview-tree-do-click): Open treemenu on mouse-3.