Enable multiselect context menus for JFX Tables/Graphical Editors and adapt AF3 implementations
In AF3, a context menu service exists such that each plugin can add contributors to the context menu of model elements. This service also handles multiselections.
For TreeViewers, the support shall be implemented in #3922.
However, for JFX-based tables and Graphical Editors, the support is for the service to build context menus is missing. To resolve the ticket, it must be clarified if the context menu service shall be used in future at all (and all context menu contributions must be migrated). If so, the base class for tables "DynamicTreeTableViewer" and the corresponding factories of the graphical editor elements must be adjusted.