editor widgets: rename to wrapper to distinguish from the real widgets

I renamed the xxxWidget classes to Wrapper, since they are not really the widgets and since they inherit the Wrapper.

The example is with the new relation reference widget where we have both widget and wrapper (see #1516).

This way, if we create custom widgets, like the color button, they can be named properly as Widget, and  also have their wrapper properly named.

The change in API does not concern python since widgets/wrappers are not available (yet).

This concerns the C++ API (since 2.4), but the chance that someone used these widgets classes in a standalone app is quite close to 0.