[341] Adapt to recent GEF Classic changes

[pcdavid]

The recent changes in question are:

• https://github.com/eclipse/gef-classic/commit/e5d6bac76ff38625b7c04877c0582452f04055ff#diff-07534b33eaa5ec93832be67ba799518320ca49589ed53976a24a66481e54e36e which changes the return type of EditPartView.getEditPartRegistry() from a plain/untyped Map into a Map<Object, EditPart>. The adaptation is trivial.
• https://github.com/eclipse/gef-classic/commit/4fce72b354db7af98cdb169d19b785a362560c81#diff-e157353a244e432d229f6caf980c2d097558bdd72baf4d3f9beb1d3162ec8bb3 is slightly more subtle as it removes a protected method we used to override (createRulerEditPart). To get a version which works with both old and new GEF versions, we now override the whole createEditPart() method instead. Hopefully its definition in GEF will not change in ways that would require us to update our own version in the future (we might not see it).

Locally these changes compile both with the 2023-03 and canary target platforms.