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/popup_theme.tres

@@ -0,0 +1,21 @@
+[gd_resource type="Theme" format=3]
+
+[sub_resource type="StyleBoxFlat" id="StyleBoxFlat_popup"]
+bg_color = Color(0.180392, 0.180392, 0.180392, 1)
+border_width_left = 2
+border_width_top = 2
+border_width_right = 2
+border_width_bottom = 2
+border_color = Color(0.36, 0.36, 0.36, 1)
+corner_radius_top_left = 6
+corner_radius_top_right = 6
+corner_radius_bottom_right = 6
+corner_radius_bottom_left = 6
+content_margin_left = 12.0
+content_margin_top = 12.0
+content_margin_right = 12.0
+content_margin_bottom = 12.0
+
+[resource]
+Panel/styles/panel = SubResource("StyleBoxFlat_popup")
+PopupPanel/styles/panel = SubResource("StyleBoxFlat_popup")