Move ms_after= to the right place in music macros (#4459)

I originally added it in #719. At that point ms_after= affected the *previous*
track due to a bug; however, I had assumed the behavior to be intentional and
compensated for it by adding ms_after= to the first track of a scenario instead
of the story screen music.

In April, I saw the documentation for ms_after= which said that it's supposed
to affect the track itself, not the previous one. Hence, I fixed the bug in
commit b023b81. However, I had forgotten that the changes I had made
in #719 depended on the old broken behavior of ms_after=.

This commit moves ms_after= to the right place, restoring the fade-out on
scenario start and removing the pointless pause after the first track.

data/core/macros/sound-utils.cfg

@@ -370,13 +370,13 @@
     # intra-scenario music, should we decide to use one.
     [music]
         name="revelation.ogg"
+        ms_after=2000
     [/music]
     [event]
         name=prestart
         [music]
             name={MUSIC}
             immediate=yes
-            ms_after=2000
         [/music]
     [/event]
 #enddef
@@ -403,14 +403,14 @@
     [music]
         name={INTRO_MUSIC}
         immediate=yes
+        ms_after=2000
     [/music]
     [event]
         name=prestart
         [music]
             name={SCENARIO_MUSIC}
             immediate=yes
             append=no
-            ms_after=2000
         [/music]
     [/event]
 #enddef