qtlib: document NAME_MAP purpose

Instead of plain CSS property names, TortoiseHg uses custom names for
controlling the appearance of its interface elements.

NAME_MAP converts those names to standard CSS properties.
8 jobs for branch/default in 40 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed pytype-py3-hg-default #105993

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passed pytype-py3-hg-stable #105992

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passed tests-py2-hg-5.5 #105986

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passed tests-py2-hg-default #105988

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passed tests-py2-hg-stable #105987

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passed tests-py3-hg-5.5 #105989

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passed tests-py3-hg-default #105991

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passed tests-py3-hg-stable #105990

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