New design for the choice page and merge the two SU wizards

[vincent-zurczak]

Currently, we have two wizards for service units, one for service providers and one for service consumers. Each wizard has its own choice page, that gathers Petals components by use case (communication protocols, service composition, etc). This page also allows to select the component version to use.

This page is ugly.
That makes the studio look like... its real age. I also think we should rely on the idea users know what is an engine and what is a connector. JBI concepts adds too much complexity. So, maybe we should hide this. Anyway, let's just keep a single wizard for service units, and let's update the choice page: connectors VS. engines, a description of the use case (provides or consumes), plus the choice of the version. That's it.