0027805: Visualization - AIS_InteractiveContext::FitSelected() is broken for global selection

AIS_Selection class has been cleaned up:
- SelectMgr_EntityOwner is now used in interface instead of Standard_Transient.
- Method ::Select() has been renamed to ::Clear() for clarity.
- Dropped unused class fields.
- Owners without Selectable are not added to the list (should never happen).

SelectMgr_SelectableObject::BndBoxOfSelected() has been modified
to properly compute bounding box of ALL selection Entities
for specified Owner (only first Entity has been used before the patch).

Methods of AIS_InteractiveContext::DisplaySelected(), ::SetSelectedAspect(),
::FitSelected(), ::SubIntensityOff() have been fixed (wrong DownCast).
AIS_InteractiveContext::AddSelect() now handles properly AIS_InteractiveObject as argument.

tests/bugs/vis/bug27805

@@ -0,0 +1,26 @@
+puts "============"
+puts "CR27805"
+puts "AIS_InteractiveContext::FitSelected() is broken for global selection"
+puts "============"
+puts ""
+
+set anImgFitAll $imagedir/${casename}_fitall.png
+set anImgFitSel $imagedir/${casename}_fitsel.png
+set anImgDiff   $imagedir/${casename}_diff.png
+
+pload MODELING VISUALIZATION
+box b 1 2 3
+vclear
+vinit View1
+vaxo
+vdisplay -dispMode 1 b
+vfit
+vselect 200 200
+vdump $anImgFitAll
+vfit -selected
+vdump $anImgFitSel
+
+set aDiffRes [diffimage $anImgFitSel $anImgFitAll 0.0 0 0 $anImgDiff]
+if {$aDiffRes != 0} {
+  puts "Error: FitSelected() does not match FitAll() for single object"
+}