This is a follow-up to #12998.

1. Remove SmartGroup
2. Replace by an ExplicitGroup

Moreover, the architecture of #12997 should be fixed

org.jabref.gui.groups.JabRefSuggestedGroups has to be moved to org.jabref.logic.groups.DefaultGroupsFactory

1. Rename org.jabref.logic.groups.DefaultGroupsFactory to GroupsFactory
2. Move the two factory methods from JabRefSuggestedGroups
3. Remove JabRefSuggestedGroups