ENH: add context menu "Mesh boolean"

Apply boolean operation on parts. No need to export stl to apply.

Change-Id: I14479d0977e5ec6f5f80f55b22ed02974b8271c7
(cherry picked from commit 14fe96b2b7529380832007a3d3eb93eb8b9a5217)

src/slic3r/GUI/GUI_ObjectList.hpp

@@ -300,6 +300,7 @@ public:
     void                merge_volumes(); // BBS: merge parts to single part
     void                layers_editing();
 
+    void                boolean();    // BBS: Boolean Operation of parts
     wxDataViewItem      add_layer_root_item(const wxDataViewItem obj_item);
     wxDataViewItem      add_settings_item(wxDataViewItem parent_item, const DynamicPrintConfig* config);