Show function values as tooltip when holding Ctrl
Show function values as tooltip when holding Ctrl

gui/pipeline_view.py:
 - QGraphicsModuleItem.__init__:
   call setAcceptHoverEvents
   New attribute function_overview
 - QGraphicsModuleItem.updateFunctionPorts:
   Create a html tooltip showing function values
 - QGraphicsModuleItem.hoverEnterEvent: *new
   Show function_overview as tooltip
   Note that this overwrites errorTrace tooltips

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