core: added File QML object type for reading and writing local files

So far only read and write on text files are implemented, can easily
be extended.

The name attribute is sanitized a bit in the setter to comply with
QFile's needs.
click_on_letter: allow to initialize levels from json file

Answers and questions are loaded from a levels file to allow for
non-latin tasks. Names of levels files are expected in the form
levels-<locale>.json and must contain valid json describing the
levels (cf. the contained default levels-en.json).

Non-default levels files are supposed to live in some to-be-defined
datapath directory (for now faked). Upon loading through a File
object JSON syntax and basic payload checks are performed. If they
fail we fall back to the packaged default file: levels-en.json
Ported click_on_letter_up
core: File: allow for setting name on-the-fly in read() and write()

... and update click_on_letter accordingly
Merge branch 'click_on_letter-rebased' of github.com:hkaelber/GCompris-qt into hkaelber-click_on_letter-rebased
ballcatch, minor use a QtObject instead of an Item
template, minor use a QtObject instead of an Item
activities, minor, use a QtObject instead of an Item