use shape-based compose sequences for hammer-and-sickle, and regroup some sequences

Merged Benno Schulenberg requested to merge benno/libx11:recomposing into master

The first patch replaces the inappropriate <C><C><C><P> compose sequence with a more fitting shape-based one: <question><backslash>, fixing issue #63 (closed) in the bargain.

The second patch groups some sequences for the same target character together, and corrects the comment for the double quote.

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