Fix custom assets not being unmounted when exiting from editor/credits

If you exited from the editor, custom assets would not be unmounted. But
I made sure to put the FILESYSTEM_unmountassets() before the
music.play(6) because otherwise the menu music wouldn't play.

You could also exit to the menu from a custom level using the
rollcredits() command, so I made sure to put a
FILESYSTEM_unmountassets() when returning to the menu from the credits
as well. I also made sure to put it before the music.playef(18) so
there's no risk of the sound effect not playing properly, or not playing
the non-level-specific one.

I added a comment to both FILESYSTEM_unmountasset()s to make sure anyone
reading the code is aware of the frame order dependency.

desktop_version/src/Logic.cpp

@@ -1,6 +1,7 @@
 #include "Logic.h"
 #include "Script.h"
 #include "Network.h"
+#include "FileSystemUtils.h"
 
 void titlelogic()
 {
@@ -107,6 +108,7 @@ void gamecompletelogic2()
         map.colstate = 10;
         game.gamestate = TITLEMODE;
         graphics.fademode = 4;
+        FILESYSTEM_unmountassets(); // should be before music.playef(18)
         music.playef(18);
         game.returntomenu(Menu::play);
         game.createmenu(Menu::gamecompletecontinue);

desktop_version/src/editor.cpp

@@ -3589,6 +3589,7 @@ void editorlogic()
         script.hardreset();
         graphics.fademode = 4;
         music.haltdasmusik();
+        FILESYSTEM_unmountassets(); // should be before music.play(6)
         music.play(6);
         map.nexttowercolour();
         ed.settingsmod=false;