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Sculpt: Dyntopo detail size edit operator

This introduces a new operator to edit the detail size of constant
detail mode in dyntopo. The way this operator works and the
functionality it provides is similar to the "Voxel size edit" operator
for the voxel remesher.

It also includes a sample mode. When pressing Ctrl, the detail size
will be sampled from the surface under the cursor, updating the
preview in real time. This allows quick resolution changes without
using the operator multiple times.

The operator is set to Shift + D, replacing the old way to change
the constant detail size of dyntopo. Shift + R will remain available to
be enabled when the voxel remesher works with dyntopo. Deciding
if both detail sizes can be unified needs a separate discussion as the
new dyntopo can work with detail sizes in parts of the mesh that can
easily crash the remesher.

The structure of these operators is similar, but the data they control,
ranges, drawing and setup functions are completely different, making it
hard to merge them into one.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9355