Add color customization to relationship lines and tables

[RamiroRodriguezC]

I've been using Mongo Modeler for a year now, especially in my university projects, and it's so great, but I feel like sometimes the lines of relationship become a" spider web" and it's a little difficult to distinguish one from the other.
I think it would be great (and I would be personally grateful :p) if the possibility of creating relationship lines of different colors was added.
Basically being able to change and choose the colors of the relationship lines freely, to avoid these "spider webs" and to be able to obtain diagrams with more color, if the user wants.
I think it would be an addition that could significantly improve the UX.
Maybe this same feature could be extended to the colors of the tables, instead of just them being green, but I think the relationship lines are where they would definitely positively impact the UX.
I think it could be done by simply adding one more parameter in the form to create relationships, where the color can be chosen from a palette or by pasting a HEX code, or something like that.

[RamiroRodriguezC]

A problem detected in my implementation

I was using my version with the color update, and I noticed a flaw in the solution that I proposed, although the .mml file does support saving the colors, and when you open it in Mongo Modeler it replicates the canvas again with the colors with which it had been saved, when a .png or .svg is generated, it is not seen with the colors that the user customized the lines

[RamiroRodriguezC]

solution proposed in #611