click on letter, play the voice letter in sync

Now we trig the play of the voice letter after the completion of the
message 'click on letter'. Before there was a fixed 1500ms but the
message is of variable length depending on the locale.

Extended GCAudio with the signals Error and Done.

src/activities/click_on_letter/ClickOnLetter.qml

@@ -232,6 +232,9 @@ ActivityBase {
         GCAudio {
             id: nextLevelAudio
             source: ApplicationInfo.getAudioFilePath("voices/$LOCALE/misc/click_on_letter.ogg")
+            onError: letterAudio.play()
+            // When this sound is complete, play the letter
+            onDone: letterAudio.playDelayed(100);
         }
 
         GCAudio {

src/activities/click_on_letter/click_on_letter.js

@@ -153,7 +153,6 @@ function initLevel() {
         items.questionItem.visible = false;
         items.letterAudio.source = GCompris.ApplicationInfo.getAudioFilePath("voices/$LOCALE/alphabet/"
                 + Core.getSoundFilenamForChar(currentLetter));
-        items.letterAudio.playDelayed(1500);
     } else {
         // no sound -> show question
         items.questionItem.visible = true;

src/core/GCAudio.qml

@@ -11,6 +11,7 @@ Item {
     property bool autoPlay
 
     signal error
+    signal done
 
     function play() {
         if(!muted) {
@@ -30,5 +31,6 @@ Item {
             console.log("error while playing: " + source + ": " + errorString)
             gcaudio.error()
         }
+        onStopped: gcaudio.done()
     }
 }