"Deep" template inheritance

Currently template files can override layout sections only one level
deep: included templates (partials) can define sections, but is
impossible to override them form templates. This commit allow partials
inherit sections from "parent" template to allow "deep" template
inheritance as seen in Twig and other non-php engines.
hasSection() method for less verbose templates

To define multiline default for a section one needs an if / else block.
hasSection() method allow to echo section if present or define a default
content with only if, templates becomes less verbose and better
readable.