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.

scroll_to_end_container.gd

@@ -0,0 +1,9 @@
+class_name ScrollToEndContainer
+extends ScrollContainer
+
+var max_value = 0;
+
+func _draw():
+	if max_value != self.get_v_scroll_bar().max_value:
+		max_value = self.get_v_scroll_bar().max_value
+		self.scroll_vertical = max_value