Rewrite "Imported Entries" feature

[koppor]

This is a follow-up to #12998.

1. Remove SmartGroup
2. Replace by an ExplicitGroup

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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

[dag2104]

/assign-me

[github-actions]

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[dag2104]

/unassign-me

[github-actions]

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