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

When Krusader users utilize some comboBoxes on the search 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 the "Contains
text" and the "Exclude Folder Names" comboBoxes on the search dialog.

Differential Revision: https://phabricator.kde.org/D28960