Duplicated labels removal

[Rdornier]

Fix issue #502 by removing duplicate labels.

1. If a new label is added, the duplicated label is filtered before adding it to the figure
2. If an existing label is edited, the duplicated label is filtered after adding it to the figure.

[will-moore]

The Undo functionality is working again, thanks. I wonder if you could clean-up the unnecessary whitespace diff lines left over since it increases the chance that you'll get a merge conflict with other PRs.

[will-moore]

This is working fine for me now, thanks.
I don't want to sound too picky, but I had a slightly hard time understanding the code, and I'm also not sure that using Object.values will always preserve the order of the items (although it seems to be working).
I just wonder, since you're filtering, if it's clearer to use the filter function like this?

            // filter unique keys only to remove duplicates
            var keys = new Set(labs.map(lbl => this.get_label_key(lbl)));
            var filtered_lbls = labs.filter(lbl => keys.has(this.get_label_key(lbl)));

Apologies: I understand that this may not be a "better" way for everyone, and I don't want to make code review and PR submission any more time-consuming than it has to be. So please feel free to ignore this suggestion if it doesn't feel better for you!

[Rdornier]

I don't want to sound too picky,

Don't worry, you have much more experience than me on Javascript !

However, I tested your changes but duplicates are not always removed. If labs contains duplicate labels, then duplicates are not removed.

[will-moore]

Sorry - I just realised that actually this code would be expected to remove ALL labels (since the keys for all of them are in that Set().
I think I must have not rebuild my JS code when I tried that and assumed my change was working!

I actually decided to google for a better way to do this (https://stackoverflow.com/questions/2218999/how-to-remove-all-duplicates-from-an-array-of-objects).
This code is concise but I'm not sure that it is very understandable?

var keys = labs.map(lbl => this.get_label_key(lbl));
var filtered_lbls = labs.filter((lbl, index) => index == keys.indexOf(this.get_label_key(lbl)));

[Rdornier]

The snippet now works correctly.
It is definitely more concise and I think it could be understandable quite easily (filtering based on the index of the first occurrence of the value)
So it should be ok 👍

[Rdornier]

It also removes the dirty Object.values(), which I don't really like as well

[will-moore]

This is working fine for me now, thanks.