Make header translatable and organize strings.php

Commit 1.
Header (dropdown menu as well as the title of the main page) are now translatable.

Greek translation has been added to that section.

Also, I organized locale/strings.php in a better way. Now strings that are missing translations are found much more easily and completely new translations are added more easily.

Commit 2.
Also, made 'seconds changed' (from ticker.js) translatable and added the Greek translation for it.