Move old slicer detection to GuiUtils

[jack-rooks]

To address some issues with old slicer plots reappearing (#2752) some extra checks were included. Most of this is in qtgui > Plotting > Plotter2D under setSlicer, but one of the attributes used for this is included in each slicer. It was brought up in review (pull request 1919) that this should probably be moved to GuiUtils (where much of this code was written based off of).

This won't be done in time for the merge, but at a later date we should generalize this code and move it to GuiUtils so that it can be used for other plotters if needed (or rewrite this to use the existing GuiUtils functionality).