Added cmPlotObjCb() function.

Changed: memset(a,0,sizeof(a))
to:      memset(a,0,sizeof(*a));

destroyed is the focused object and updates p->fop accordingly.

Rewrote cmGr.c::cmGrEvent()
Callback cmGrIsInsideObj() now must evaluate both if the mouse point is
inside the object and also if the object can handle the event. This
allows the actual plot object which can handle an event to be identified
by cmGrEvent().

_cmGrPlotObjEvent() still needs to be updated to account for these changes.