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
Actually I think stubbing like this is enough (as we stub the "entry point" of the external module)