Separate widget_addon in order to have both prefix and suffix addon

I propose you this PR in order to have both prefix and suffix addons, as related in ticket #518.

I split `widget_addon` in two options, `widget_addon_prepend` and `widget_addon_append`.

This cause a BC break, so I kept back the widget_addon option and throw a deprecated error to remove after, but I don't know if is necessary because the 2.2.x branch is currently on alpha.

By the way, if you accept this PR, can you please push a new alpha or stable version of your 2.2.x branch please ?

Don't hesitate to ask me if you have some question, remarks or something else !