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/integration/test_music_list_panel.gd

@@ -37,7 +37,7 @@ func test_starts_empty() -> void:
 func test_refresh_shows_one_row_per_song_in_order() -> void:
 	_panel.refresh()
 
-	assert_array(_rows().map(func(row: SongListObject) -> int: return row.song_object.song_number)) \
+	assert_array(_rows().map(func(row: SongRow) -> int: return row.song_object.song_number)) \
 			.contains_exactly([0, 1, 2])