[ENHANCEMENT] Save modified trajectory visualisation settings

[MBartkowiakSTFC]

Is your feature request related to a problem? Please describe.
At the moment it is possible to change the radii of atoms or the colours of atoms and background. However, the changes are not permanent and can be undone by switching to another trajectory.

Describe the solution you'd like
We can use the configuration file of the Trajectory tab to store the changes.

Describe alternatives you've considered
Should this be done for all trajectories, or per trajectory? Should the changes be saved in the trajectory file?

Additional context
The main question is if the users want to change the appearance of all their trajectories at the same time, or to make each one of them different.