Add ability to collect methods in _UI assemblies and expose them in the UI ribbon like we do for the engine

[Arnaud Declercq]

Description:

As per title.
Obviously, the categories/classes will be different from the ones we find in the Engine so we will need a workshop around that.
Otherwise, the concept is very much the same as the Engine with a pre-define set of static classes will contain the groups of methods to expose in the ribbon.
The possible difference here is to accommodate for the possibility that each UI will need a different set of categories. So we might want to tag the classes with a special attribute to allow for extra flexibility if needed. (The attribute would need to contain the icon as well then).

@adecler and @pawelbaran to organise a workshop in the coming milestone.