Check if each symbol has a parent

When manually editing symbols, or using tooling that is not KiCad itself, symbol files might get corrupted. One possible fault is to have a symbol that is derived but the parent does not exist. Unfortunately this code will not trigger if only the sorting is wrong. KiCad expects the derived symbols to be later in the file than the parent. If that is not the case, it fails to load the derived symbol. See !4215 In order to also detect that, we need line number information. Which is not easily available in the symbol parser.