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)
I really don't like the exception here.
How about, let's freeze these keys anyway, but create our own space/enter key objects w/ modification in `_updateCurrentPage`, make the modification and freeze it? This implies we will no longer use Object.create() to link the original key and the modified key.