UI: top-bar reorganization

This hides the top-bar by default for everything
besides paint/sculpt workspaces.

- Use the top-bar mainly for active tool settings &
  popovers panel options.
  (transform / snap settings are an exception for this convention).
- Only show the top-bar (by default)
  in paint work-spaces (sculpt / texture-paint / grease-pencil).
- Add an active-tool panel to the sidebar.
- Split 3D view tabs into (Item / Tool / View).

D4721 with minor changes.

Further work is needed for the top-bar and image-editor.

release/scripts/startup/bl_ui/space_toolsystem_common.py

@@ -566,8 +566,13 @@ class ToolSelectPanelHelper:
             context, layout,
             *,
             show_tool_name=False,
+            tool_key=None,
     ):
-        space_type, mode = ToolSelectPanelHelper._tool_key_from_context(context)
+        if tool_key is None:
+            space_type, mode = ToolSelectPanelHelper._tool_key_from_context(context)
+        else:
+            space_type, mode = tool_key
+
         if space_type is None:
             return None
         item, tool, icon_value = ToolSelectPanelHelper._tool_get_active(context, space_type, mode, with_icon=True)