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 files, tests, or entry points are named. Start by reviewing the migration referenced in #486 and tracing uses of the old ResourceManager; done means the relevant methods are marked for removal, user-code references produce warnings, and the later removal remains planned for a future release.

Written by the indexing model from the issue text.