Change the colour of messages, comments and dividers with context menu

[MrCoder]

Colouring elements on the diagram is a much wanted feature. ZenUML DSL and the renderer already support colouring participants, messages, comments and dividers. However, they are not well documented and hard to remember. It would be better if we can provide a context menu item.

Design is required.

First level context menu

When a message is selected, we need to be able to change the colour of message and comment separately (if there is a comment).

Second level context menu