Allow overriding of InteractivePlot elements

[joschaschmiedt]

Making InteractivePlot::plot as well as XYLine::draw virtual allows re-using them in custom visualizer plugins. This would be very useful for adjusting the way how things are displayed.

[anjaldoshi]

Thanks for making these updates. Making InteractivePlot::plot and XYLine::draw virtual will definitely make it easier to customize how plots are rendered in plugins that rely on the InteractivePlot class.

While this does introduce a breaking change to the PLUGIN_API that would prevent existing plugins using InteractivePlot from loading, it's currently only used by the Spectrum Viewer plugin which we can re-deploy for v1.0 after merging these changes. So we’re fine to go ahead with the merge.