0026159: Visualization - revise tolerance implementation for selection

Max tolerance is now applied to all objects and entities by default;
if there is an entity with a lower tolerance, selecting frustum will be recalculated for it.
The ability to set sensitivity for each entity individually is moved to protected section of SelectBasics_SensitiveEntity
SetPixelTolerance from AIS_InteractiveContext now sets custom tolerance that is applied to all sensitives.
Added -unset option to vselprecision command to disable custom tolerances.
Test case for issue #26159

src/SelectMgr/SelectMgr_BaseFrustum.hxx

@@ -88,6 +88,10 @@ public:
 
   virtual NCollection_Handle<SelectMgr_BaseFrustum> Transform (const gp_Trsf& /*theTrsf*/) { return NULL; }
 
+  //! IMPORTANT: Makes sense only for frustum built on a single point!
+  //! Returns a copy of the frustum resized according to the scale factor given
+  virtual NCollection_Handle<SelectMgr_BaseFrustum> Scale (const Standard_Real /*theScaleFactor*/) { return NULL; }
+
   //! SAT intersection test between defined volume and given axis-aligned box
   virtual Standard_Boolean Overlaps (const BVH_Box<Standard_Real, 3>& theBndBox,
                                      Standard_Real& theDepth);