Design: move bodies into their own Parts list (Onshape-style)

Bodies were appended INSIDE the feature tree, after the features, and only when
there was more than one body. Two consequences:

- With a single body — sketch + extrude, the common case — no body row existed
  at all, so the solid could not be selected from the tree. That also blocked
  Move (it requires a selected body), the show/hide eye and every body-targeted
  op; the viewport was the only way to select.
- With several features the Bodies group was pushed past the tree's auto-sized
  height (capped at 9 rows) and clipped out of view, so bodies became
  unreachable as history grew.

Bodies now live in their own list under the feature tree, mirroring Onshape's
Features + Parts split that the rest of the tab already follows. The list is
hidden while empty, sizes to its content (scrolls past 6), keeps the selected
row across a recompute, and greys hidden bodies as before. Selecting in either
list clears the other, so only one thing is ever "the target".

Verified on :10: single body -> Body 1 listed, selectable, Move opens the gizmo
on it (previously impossible); two imported solids -> both listed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/DesignPanel.hpp

@@ -425,6 +425,8 @@ private:
     // integer row indices via tree_selection()/set_tree_selection(); m_tree_items
     // maps feature order -> tree node, rebuilt by refresh_tree().
     wxTreeCtrl*               m_tree{nullptr};
+    wxTreeCtrl*               m_parts{nullptr};        // Bodies list under the feature tree
+    wxStaticText*             m_parts_label{nullptr};  // its "Bodies" caption (hidden when empty)
     wxImageList*              m_tree_images{nullptr};
     std::vector<wxTreeItemId> m_tree_items;
     // Parts list: tree rows for each body (parallel to m_doc.bodies). Selecting one
@@ -461,6 +463,7 @@ private:
     void on_set_body_color();             // Color tool: pick a per-body display colour override
     int  tree_selection() const;          // selected feature row, or wxNOT_FOUND
     int  tree_body_selection() const;     // selected Parts-list body index, or -1
+    void refresh_parts();                 // rebuild the Bodies list under the feature tree
     void set_tree_selection(int row);
     static int tree_icon_for(CadFeatureType t);