findit, removed a reference to main which creates undef

When we leave an activity main is undef and we are still bind on it.
advanced_colors, minor rework of the background
core audio, fix audio to work after an invalid file

When an invalid file is play we are in a state where we stay
locked. This patch is tested on Linux and Android.

When an invalid audio file is played, the state of the Audio item
on Android and on Linux is different.
erase, removed credits as they are uncomplete and outdated.
main, enabled intro music and intro voice welcome to GCompris.
findit, minor graphic improvement
missing letter, add voice play of the selected letter