Sort the scripts and scenes into folders

main/ holds the main window and top bar, quiz/ the quiz and its parts,
player/ the music player, popups/ every popup and what they share, data/ the
songs, players, playlist and settings, server/ the MusicServer client,
common/ the list row and GameLog, and themes/ the themes and style boxes.

tests/unit/test_search.gd

@@ -1,7 +1,7 @@
 extends GdUnitTestSuite
 ## How the search popup finds and orders matching soundtracks.
 
-const SearchWindowScript: GDScript = preload("res://search_popup.gd")
+const SearchWindowScript: GDScript = preload("res://popups/search_popup.gd")
 
 var _search: Control