Sculpt/Paint: Automatically switch tools when activating brushes

As discussed in #128066, we want that activating a brush makes sure the
most appropriate tool is activated. For example activating an eraser
brush should activate an eraser tool, if available. This makes the
tool and brush binding two ways, which is more clear. Plus it means that
users don't have to care about tools at all if they don't want to, they
can just keep switching brushes and Blender takes care of the tool.

Implementation wise, this works by letting a Python operator lookup a
tool for a given brush type name, when activating a brush.

Design Task: https://projects.blender.org/blender/blender/issues/128066

scripts/startup/bl_operators/wm.py

@@ -2390,6 +2390,60 @@ class WM_OT_tool_set_by_index(Operator):
             raise Exception("Internal error setting tool")
 
 
+class WM_OT_tool_set_by_brush_type(Operator):
+    """Look up the most appropriate tool for the given brush type and activate that"""
+    bl_idname = "wm.tool_set_by_brush_type"
+    bl_label = "Set Tool by Brush Type"
+
+    brush_type: StringProperty(
+        name="Brush Type",
+        description="Brush type identifier for which the most appropriate tool will be looked up",
+    )
+
+    space_type: rna_space_type_prop
+
+    def execute(self, context):
+        from bl_ui.space_toolsystem_common import (
+            ToolSelectPanelHelper,
+            activate_by_id
+        )
+
+        if self.properties.is_property_set("space_type"):
+            space_type = self.space_type
+        else:
+            space_type = context.space_data.type
+
+        tool_helper_cls = ToolSelectPanelHelper._tool_class_from_space_type(space_type)
+        # Lookup a tool with a matching brush type (ignoring some specific ones).
+        tool_id = "builtin.brush"
+        for item in ToolSelectPanelHelper._tools_flatten(tool_helper_cls.tools_from_context(context, mode=context.mode)):
+            if item is None:
+                continue
+
+            # Never automatically activate these tools, they use a brush type that we want to use
+            # the main brush for (e.g. grease pencil primitive tools use 'DRAW' brush type, which
+            # is the most general one).
+            if item.idname in {
+                    "builtin.arc",
+                    "builtin.curve",
+                    "builtin.line",
+                    "builtin.box",
+                    "builtin.circle",
+                    "builtin.polyline",
+            }:
+                continue
+
+            if item.options is not None and ('USE_BRUSHES' in item.options) and item.brush_type is not None:
+                if item.brush_type == self.brush_type:
+                    tool_id = item.idname
+                    break
+
+        if activate_by_id(context, space_type, tool_id):
+            return {'FINISHED'}
+        else:
+            self.report({'WARNING'}, rpt_("Tool {!r} not found for space {!r}").format(tool_id, space_type))
+            return {'CANCELLED'}
+
 class WM_OT_toolbar(Operator):
     bl_idname = "wm.toolbar"
     bl_label = "Toolbar"
@@ -3597,6 +3651,7 @@ classes = (
     WM_OT_url_open_preset,
     WM_OT_tool_set_by_id,
     WM_OT_tool_set_by_index,
+    WM_OT_tool_set_by_brush_type,
     WM_OT_toolbar,
     WM_OT_toolbar_fallback_pie,
     WM_OT_toolbar_prompt,