Bug 1072191, 1074613 - Layout normalization follow-ups

- |LayoutLoader._normalizeLayout| is now delegated onto LayoutNormalizer
-- Alternativs chars normalization codes are moved to LayoutNormalizer
- LayoutNormalizer is no longer a singleton (better pattern since it's only used locally)
- textLayoutOverwrites is now normalized
Merge pull request #25209 from mnjul/bug_1074613_normalize_alt_char_and_overwrite_chars

Bug 1072191, 1074613 - Layout normalization follow-ups. r=timdream