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.

tests/unit/test_search.gd

@@ -1,6 +1,6 @@
 extends GdUnitTestSuite
 
-const SearchWindowScript: GDScript = preload("res://SearchWindow.gd")
+const SearchWindowScript: GDScript = preload("res://search_popup.gd")
 
 var _search: Control