qtgr: Handle events for multiple plots in one Widget.

Support events for multiple plots with/in different viewports.

Change-Id: If7aa2a250f9f579a9aea40a9ed9cf5a0d79217e2
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/127
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>