Sculpt: Change symmetrize merge threshold and expose in workspace panel

The sculpt symmetrize operator's merge threshold now defaults
to 0.0005 instead of 0.001, which tends to be a bit too big for
metric scale.

Also changed its step and precision a bit to be more usable.

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -1020,6 +1020,7 @@ class VIEW3D_PT_sculpt_symmetry(Panel, View3DPaintPanel):
 
         layout.prop(sculpt, "symmetrize_direction")
         layout.operator("sculpt.symmetrize")
+        layout.prop(WindowManager.operator_properties_last("sculpt.symmetrize"), "merge_tolerance")
 
 
 class VIEW3D_PT_sculpt_symmetry_for_topbar(Panel):

source/blender/editors/sculpt_paint/sculpt_ops.c

@@ -246,15 +246,17 @@ static void SCULPT_OT_symmetrize(wmOperatorType *ot)
   ot->exec = sculpt_symmetrize_exec;
   ot->poll = sculpt_no_multires_poll;
 
-  RNA_def_float(ot->srna,
-                "merge_tolerance",
-                0.001f,
-                0.0f,
-                FLT_MAX,
-                "Merge Distance",
-                "Distance within which symmetrical vertices are merged",
-                0.0f,
-                1.0f);
+  PropertyRNA *prop = RNA_def_float(ot->srna,
+                                    "merge_tolerance",
+                                    0.0005f,
+                                    0.0f,
+                                    FLT_MAX,
+                                    "Merge Distance",
+                                    "Distance within which symmetrical vertices are merged",
+                                    0.0f,
+                                    1.0f);
+
+  RNA_def_property_ui_range(prop, 0.0, FLT_MAX, 0.001, 5);
 }
 
 /**** Toggle operator for turning sculpt mode on or off ****/