Wordlist new XML properties for gletters & wordsgame

- Wordlist levels have 3 new, optional parameters:
-- fallspeed: how fast the words fall
-- speed: how fast a new item is dropped after the last one
-- sublevels: how many items are in the level

- gletters:
-- use the new parameters
-- display bonus at end of level

- wordsgame:
-- use the new parameters
-- display bonus at end of level
-- levels now start with a defined speed + fallspeed
   when the user clicks through the levels
falling letters: activated letter voice is now by default
falling letter: improved level sequence for french and english.
core sound: fixed a race around background music

This commit fixes a race condition that happens when we
start GCompris and enters an activity that mutes the
background music before the intro music is completed.

In this case, the intro music was not muted as expected.
hangman, minor translator comment update.