core: added Wordlist.qml for loading an maintaining GCompris wordlists

The Wordlist element reads GCompris' wordlists from json files. It
expects a json format that is semantically identic to the former xml
format of the Gtk+ version and scetched briefly in comments.

This adds also a converter script (tools/wordlist-xml2json.pl) that
transforms Gtk+ wordlist files from xml to the new json format.
Ported Gletters activity

First version without Android support.

Uses Wordlist.qml for loading wordlist files. No support for multichar
words yet, will come soon.
Merge pull request #23 from hkaelber/integration-master

Gletters: first version