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
MDL-38309 Added comments on overwriting functions in themes

Defined function tabree() as final so themes do not override it by mistake.
Added comments to other similar shortcut functions that themes should not override them (can not declare them final
for backward compartibility)