Add/Remove curves from plot

Logic for adding and removing curves from BECWaveformPlot is to be reviewed. We need more checks to ensure that error popups are minimised, and/or a strategy to add plots from comboboxes for building larger GUIs. The following use cases exist:

• append a new curve to the current plot
• exchange the current curve of the plot (plotting a unique signal (scan_segment & async only probably))
• the exact same for DAP curves

We should discuss this in more detail again with the different use cases and desired behaviour.