Make language menu work.

The language is now part of the URL:

    /et/contact
    /en/contact

The existing baseUrl() method takes the language into account,
while the new rootUrl() gives the true root of the site and is
therefore useful for referencing assets (JS, CSS, images).
Avoid a link to /front-page.
Include page title to translatable strings.
Shorten HTML source lines in tpl/index.php
Include slider JS only in front page.

Rename main.js to slider.js to make things more clear.