core, audio refactoring

We now have 2 global audio items, one for the voices, one for the effects.

They both can be mutted from our configuration dialog

They are both passed to the ActivityBase of the activity and they must be used
there instead of creating a GCAudio in the activity itself.

This patch does not include the refactoring of all activities.
all, audio refactoring to use the global audio items
audio and click_on_letter

Added a silence(duration_ms) function in GCAudio to force
a silence before the play of the next audio file

Fixed references to isAudioEnabled

Changed click_on_letter to use the global GCAudio and the new silence
function (not very well tested)