[Paint] enhancement of label color with list of current labels

[mathildemerle]

In Paint, it's not very clear that you can add different type of labels with different mask values, on a same drawing.

This is because when you change the color of a label without changing the value, the drawing keeps the original color (which could be a bug). The user needs to change the values as well.

It would be interesting for users to have a list of labels with "+" and "-" buttons to remove or add ones, as done in PolygonRoi toolbox.

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