Create extensions points

[ecarriou]

It is possible to extend System Designer by adding systems in addons directory and then build System Designer, but:

• we need to provide an extension manager to remove / add on the fly extensions
• extensions must be for toolbar / menubar (i.e. create buttons and their behaviors when we click on it, create tabs and their behaviors when we click on it)

[ecarriou]

You can extend System Designer by composing other systems with its systems. In that case, these systems are called addons.

To extend System Designer, just copy and paste a system into the /addons directory and run npm run web.