Rename layout.html to page.html

Issue #71.
Clean up test files
Remove header.html

Issue #71.
Rename header-nav to navbar

Issue #71.
Rename title.text to title.heading

Issue #71.
title.html: Add support for displaying results

Issue #71.
Remove footer-nav and replace with footer.html

Issue #71.
Prefix all component templates with '_'

These templates shouldn't be rendered directly.

Issue #71.
Revert "Clean up test files"

This reverts commit c64389491c839aa12e722025d7b9e663489d4800.
Remove test template

Issue #71.
template.php: Check that template is not private

...else  print an error.

Issue #71.
template.php: Define template constants

Issue #71.
template.php: Add a template_exists() check

Issue #71.