Allow to define a resolution order between feature and group for macro

While writing some documentation regarding groups and features in wikimacro I discovered a limitation. Right now we have the following:

Let's say the developer needs to represent a reference which can be either a URL or an internal representation which is composed of a string and a type. The developer wants the following:

• the user should be able to chose between a URL or an internal representation
• when the internal representation is chosen, both the type and the string should be filled

For doing that, we need 3 parameters:

• the URL parameter, which is bound to a feature "reference"
• the string internal representation parameter, which is also bound to a feature "reference" but also associated to a group "internalrepresentation"
• the type internal representation parameter which has characteristic as the previous: it's bound to a feature "reference" but also associated to a group "internalrepresentation"

And now, imagine we want to have systematically a suffix for that reference, and since it's part of the reference we want that suffix to be grouped.
So now we want a group containing the suffix and the feature choice we had before, we're adding one level to the tree.

Right now, this is not properly handled it seems we cannot achieve this representation.