Implement Shift+Del to remove cruft from the history of: the "Search for" comboBox on the "Locate" dialog

When Krusader users utilize the "Search for" comboBox on the "Locate" 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 "Search for" comboBox on the "Locate" dialog.

Differential Revision: https://phabricator.kde.org/D29000