Enable colouring of the nodes in the diagram

[valeriosantarelli]

Add a toggle button on the menu which automatically colors all the concept/role/attribute/individual nodes in the diagrams.

For the colors, we can try with the ones in the file in attachment (first five colors in the canvas).

We could think about also changing the background color to something cream-colored (which might look well), to provide a completely different color scheme, like what some IDEs do, but that can be done later.