Remove usage of deprecated Observable classes

[pcdavid]

Starting in 2019, the Eclipse Platform has marked several databinding classes as deprecated for removal. See #546820 and the migration guide.

This includes:

• org.eclipse.jface.databinding.viewers.ViewersObservables
• org.eclipse.jface.databinding.swt.SWTObservables
• org.eclipse.core.databinding.beans.BeansObservables
• org.eclipse.core.databinding.beans.PojoObservables
• org.eclipse.ui.databinding.WorkbenchObservables

They will be completely removed in 2022-09.

GMF Runtime uses some of these, so must be adapted.

From a quick read of the comments on bug #546820, it seems that the replacements have only been complete starting from around 2020-06 (not sure yet). Depending on how GMF Runtime uses these classes, if we want to be compatible with 2022-09 where these will no longer be available, it might mean we need to drop support for platforms older than ~2020-06 (so that we can use the alternatives).

[pcdavid]

As part of #6, support for platforms older than 2020-09 have been removed.
It seems that platforms supported in the upcoming version 1.15.0 now include complete replacements for the deprecated APIs, so if this is confirmed (and if time permits) this could be included in 1.15.0.