GUI: Improve usability of beam status graph

The traces for TS1, TS2 and Synchrotron current in the beam status graph overlap heavily and it is therefore hard to read any one of them individually. As a user I would like access to a feature that allows me to make the traces more discernible.

Suggestions:
1. Implement control to turn individual traces on/off
2. Implement control to highlight individual traces (e.g. bring selected trace to foreground, colour the selected trace and discolour the rest)
3. Allow for rescaling of the axes

Notes
The choice should be preserved when switching between plot periods (as per #1324)