Bug 1072191, 1074613, 1072186 - 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
- Layout keys are frozen after normalization (in normalizer) or creation (in LayoutManager)
We'll re-insert the frozen enterKey back to |keys| so it's necessary to keep its position