Improve implementation of linked plot bounds and cursors

[s-nie]

The linked bounds I originally introduced and the cursor groups that have since been implemented in a similar fashion have some drawbacks. Users have to create and store them between frames, which goes against egui's approach to store data between frames in its own memory. The stored Rc in these groups also puts some avoidable limitations on user code, e.g. parallelism.
I've also noticed a regression in the plot bound handling. Changing the plot bounds manually when linked axes are active makes the plot non-interactive.

I have a PR ready to addresses both of those issues.