0027969: Visualization - add interfaces to access selecting volumes from SelectMgr_SelectingVolumeManager

- getter for selection frustum computed during last run of selection mechanism was added to SelectMgr_SelectingVolumeManager;
- getters for frustum planes were added to SelectMgr_BaseFrustum and its successors;
- an API for applications to get frustum planes was added to SelectBasics_SelectingVolumeManager.

src/SelectBasics/SelectBasics_SelectingVolumeManager.hxx

@@ -108,6 +108,10 @@ public:
   //! correspondingly) onto far view frustum plane
   virtual gp_Pnt GetFarPickedPnt() const = 0;
 
+  //! Stores plane equation coefficients (in the following form:
+  //! Ax + By + Cz + D = 0) to the given vector
+  virtual void GetPlanes (NCollection_Vector<NCollection_Vec4<Standard_Real> >& thePlaneEquations) const = 0;
+
 protected:
   SelectionType myActiveSelectionType;      //!< Active selection type: point, box or polyline
 };