Implement Shift+Del to remove cruft from the history of: some comboBoxes on the Synchronizer dialog

When Krusader users utilize some comboBoxes on the Synchronizer dialog: they
often see bad suggestions because there are obsolete entries, mistakes and so
on, but normal Krusader users can't remove those entries. For example, web
browsers have Shift+Del in order to remove the thwarting entries.

If the KrHistoryComboBox class is used, then Krusader users are able to delete
wrong entries from history lists, and therefore they can remove those bad
suggestions that they were seeing.

ADDED: Shift+Del removes undesired entries from the history of some comboBoxes on the Synchronizer dialog.

Differential Revision: https://phabricator.kde.org/D28963