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.