Mark support for old ResourceManager for removal

As part of migrating to the JFace resource manager as part of #486, I think we should also make preparations to eventually get rid of the old implementation.

I propose that this removal is done via the following steps:
- Mark the corresponding methods in the WindowBuilder projects as `@Deprecated(forRemoval=true)`
- Adapt the builder to create a warning, whenever the old resource manager is referenced in user code
- Remove the old implementation in an unspecified, future release.

By creating a warning, we let the user know about the planned removal. And by waiting a few releases, we give them time to adjust their code, before we eventually drop the support.

No se nombran archivos, pruebas ni puntos de entrada. Empieza revisando la migración referenciada en #486 y rastreando los usos del antiguo ResourceManager; se considera terminado cuando los métodos relevantes estén marcados para su eliminación, las referencias en el código de usuario produzcan advertencias y la eliminación posterior siga planificada para una versión futura.

Escrito por el modelo de indexación a partir del texto del issue.