Musics and sounds, transition between musics and complet sounds

All over the game, whenever there is a transition between different musics, or when a music must be interrupted (for example at the end of a combat), there should be a fade in the sound (the sound level is attenuated until it is no longer heard) before switching to the other music and when the music needs to be stopped.

Also when a sound is played, it must be fully played before the next action is taken.

In fheroes2, for example during combat, when the last opposing creature is decimated, the creature's sound may not be fully played and may be cut off abruptly when the end of combat window is displayed. The end of combat window should only be displayed when all combat sounds have been fully played. The music of the combat is also cut off abruptly when the end of combat window is displayed.

Related issue: https://github.com/ihhub/fheroes2/issues/1979.