Preload songs only after the song list has loaded

At startup and after resetting the playlist, loading the song list and
preloading the first song started at the same time. When the list
answered last, it cleared the playlist and dropped the preloaded song.
Preloading now starts once the list has loaded, and loading the list is
retried if it fails.

tests/integration/test_main_window_startup.gd.uid

@@ -0,0 +1 @@
+uid://sfwrqidmuj04