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.

player_name_field.gd

@@ -0,0 +1,39 @@
+class_name PlayerNameField
+extends TextEdit
+
+signal enter_key_pressed
+signal close_pressed
+
+@export
+var LIMIT: int = 10
+var current_text = ''
+var cursor_line = 0
+var cursor_column = 0
+
+func _ready():
+	text_changed.connect(_on_text_changed)
+
+func _input(event):
+	if event is InputEventKey and event.pressed:
+		if event.alt_pressed && event.keycode == KEY_A:
+			close_pressed.emit()
+		if has_focus():
+			if event.keycode == KEY_ENTER || event.keycode == KEY_KP_ENTER:
+				enter_key_pressed.emit()
+
+func _on_text_changed():
+	if text.contains("\n"):
+		text = text.replace("\n", "")
+	
+	var new_text : String = text
+	if new_text.length() > LIMIT:
+		text = current_text
+		# when replacing the text, the cursor will get moved to the beginning of the
+		# text, so move it back to where it was 
+		set_caret_line(cursor_line)
+		set_caret_column(cursor_column)
+
+	current_text = text
+	# save current position of cursor for when we have reached the limit
+	cursor_line = get_caret_line()
+	cursor_column = get_caret_column()