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
I actually don't like the way `MockHelper` works and I don't think we need to create `MockLayoutNormalizer` in this case.

A simple object stub should allow you to extract the layout loaded (and sent to the normalizer) w/o additional code. e.g.,

    stubLayoutNormalizer = this.sinon.stub(LayoutNormalizer.prototype);
    this.sinon.stub(window, 'LayoutNormalizer').returns(stubLayoutNormalizer);