NOTE: This is committed to the 3.3 branch as part of D15606, which we
decided should go to this release still (by Bastien, Dalai and me). That
is because these are important usability fixes/improvements to have for
the LTS release.
This basically lets the UI use the constraint or modifier icon, whenever
refering to constraints/modifiers via RNA pointers. Used by D15606, so
that the "Modifiers" tree element to group the individual modifiers
together gets the right icon without hardcoded handling.
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