add configuration to disable play at startup (BUG-408134)

I added an entry in the configuration dialog to choose if the playback should start immediately at startup.

This could fix https://bugs.kde.org/show_bug.cgi?id=408134

It should already work.

Only thing remaining to do is to fix an autotest but I would like to be sure that the current implementation is ok before doing it.

Another solution would be to use a toggle outside the app configuration, that would be persisted in persistentSettings.audioPlayerState (in ElisaMainWindow.qml) but I think that such an important setting and also a setting that you don't change often (if you don't like autoplay you don't need to always have a button or something in front of you to change the behaviour) is best suited for application settings rather than always present on screen.