Name popups *Popup and give scripts, scenes and classes matching names

Every popup is now called *Popup instead of *Window in its file, root node,
node name, class and variable, and PopupWindow becomes BasePopup. Player is
PlayerRow, SongListObject is SongRow and Character is CharacterCard. Script
files are snake_case and scene files PascalCase, each matching its class or
root node, and LightGrayTheme.tres is light_gray_theme.tres.

settings.gd

@@ -0,0 +1,38 @@
+extends Node
+
+const MAX_NUMBER_OF_PLAYERS: int = 7
+
+var default_path: String = "https://music.sanplex.xyz"
+var selected_server: int = 0
+
+var is_local: bool = false
+
+var is_debug: bool = false
+
+var stop_after_current: bool = true
+var auto_repeat_song: bool = true
+var hide_next_track: bool = true
+var add_to_stats: bool = true
+var use_low_played_mode: bool = false
+var winning_score: int = 20
+var fullscreen: bool = false
+var play_local: bool = false
+var inspiration_list_speed: int = 1
+var hide_for_seconds: float = 5.0
+var hide_beginning: bool = false
+var hide_length: bool = false
+var hide_ticks: bool = false
+
+var player_array: Array[PlayerObject]
+var edit_players: bool = false
+var currently_syncing: bool = false
+var character_select_open: bool = false
+
+## The game version, set in Project Settings > Application > Config > Version.
+## Releases are made from it, see RELEASING.md.
+var version: String = ProjectSettings.get_setting("application/config/version", "")
+
+func delete_children(node: Node) -> void:
+	for n: Node in node.get_children():
+		node.remove_child(n)
+		n.queue_free()