By trying to keep strict knowledge of selected object in canvas I
        broke the functionality of TView manipulation and caused too many
        enter/leave events to be passed during mouse motion.

        * gpad/inc/TCanvas.h:
        * gpad/src/TCanvas.cxx:
        Removed member 'Bool_t fSelectedChanged' and added 'TObject
        *fClickSelected' with get/set methods. Added signal-emitting 'void
        Cleared(TVirtualPad *pad)'.

        In 'HandleInput()' the code restoring the selected object if the
        Selected signal was not emitted has been removed - this value is
        now available via 'GetClickSelected()' function. As restoring of
        values is no longer necessary local TObjectSpy variables have been
        replaced with ordinary pointers.

        The Cleared signal will be used in TGedEditor to detect clearing
        of a pad (which also happens on many object->Draw() invocations).

        * gpad/src/TPad.cxx:
        In 'Clear()' emit 'TCanvas::Clear(this)' instead of
        'Selected(this, this)'.
        In 'RecursiveRemove()' check also if deleted object is
        fCanvas->fClickSelected() and resets it to 0.


git-svn-id: http://root.cern.ch/svn/root/trunk@16116 27541ba8-7e3a-0410-8455-c3a389f83636