MDL-38309 move HTML generation from print_tabs() to renderer

- created renderable object tabtree that represents the tree of tabs, extends tabobject
- created core_renderer::tabtree(), and supporting core_renderer::render_tabtree(), core_renderer::render_tabobject()
- change print_tabs() to use renderer function, deprecate supporting not needed functions
MDL-38309 Substitute usage of print_tabs() with renderable tabtree