GUI: spectra plots #73

In the UI side of the datastreaming OPIs, make the spectra plots screen more similar to the existing DAE spectra plots screen, by:

• Displaying 4 plots simultaneously with independent settings
• Adding a new OPI for the "combined spectrum plot" view
• Adding support for "counts" and "counts/us" modes
• Changing labels/units etc in different modes

The underlying PVs for this functionality should already exist - https://isiscomputinggroup.github.io/kafka_dae_diagnostics/pvs.html