Keep scan plots in a single plot

[rprospero]

It's possible to get multiple plots going with the scanning system, which can end with the user being forced to scroll through the plots to find the one that they are looking for. The scanning subsystem should ensure that there are no other plotting windows open.

[John-Holt-Tessella]

On CRISP they were quite pleased to have multiple plots open when aligning the beamline. They could look back at previous plots to see what happened last run (e.g. when aligning angle and height of detector). Maybe we should set a user preference for this instead of switching it off.
We did find that the user runs into a problem when older plots are not closed because it fails to plot the latest plot if the buffer is full but we have a fix for that which monitors the plots going to the GUI and closes them. (ISISComputingGroup/IBEX#4308)