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.

themes/neutral_gray.tres

@@ -0,0 +1,4 @@
+[gd_resource type="StyleBoxFlat" format=3 uid="uid://blf3g88mugovr"]
+
+[resource]
+bg_color = Color(0.257813, 0.257813, 0.257813, 1)