Implement Shift+Del to remove cruft from the history of: the "Command line switches" comboBox on the "Pack" dialog

When Krusader users utilize the "Command line switches" comboBox on the "Pack"
dialog: they often see bad entries because there are obsolete ones, 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.

With a KrHistoryComboBox, 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 the "Command line switches" comboBox on the "Pack" dialog.

Differential Revision: https://phabricator.kde.org/D28999