General spacing rules

Created by: fred-wang

Follow-up of #30 (closed), this is an entry to discuss spacing rules:

• For now, the implementation note only says that only operator lspace/rspace and italic correction adds spacing in row-like elements: http://www.mathml-association.org/MathMLinHTML5/S3.html#SS3.SSS1
• TeX spacing is mentioned here: http://www.mathml-association.org/MathMLinHTML5/S2.html#SS2.SSS2
• Gecko's non-standard rules: http://hg.mozilla.org/mozilla-central/file/tip/layout/mathml/nsMathMLContainerFrame.cpp#l1058