Feature Request: Add julia API to modify existing plots shown in different cells

[disberd]

Modifying existing plots from different cells and from the JS side is useful many times, especially if the plot is outputting some variable that is bound on julia using @bind.

Modifying the plot via JS does not trigger a @bind reset and so is preferable.

Main idea to do this is to add a flag to the PlutoPlot object (default to true) that enforces probably via the plot id in JS that this plot is only rendered once. This way one can pass the julia plot object to a julia function and be sure that not more than one plot exists for that specific julia object.