core, add support to display the Qt version

It is the version of Qt we run with, not the one we compiled with.
core audio: removed autoplay property in our Audio item

This property is useless in our case and it create a bug on Android
with Qt5.4 only the first audio is played.
click_on_letter activity, no more play the audio on error

It is too confusing for children to have the wrong letter being
played when clicked. The children then forget what is expected.