Finished concept of gizmos with no toolbar icon:

on_is_selectable and on_is_activable functions are now completely independent,
the former says if there shall be an icon in the left toolbar, the latter
says if the gizmo can be activated (by a shortcut or GLGizmoManager::open_gizmo)

src/slic3r/GUI/Gizmos/GLGizmoMmuSegmentation.hpp

@@ -109,6 +109,7 @@ protected:
     std::string on_get_name() const override;
 
     bool on_is_selectable() const override;
+    bool on_is_activable() const override;
 
     wxString handle_snapshot_action_name(bool shift_down, Button button_down) const override;