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.

Open Krusader. Over a file: press Alt+Shift+P to open the "Pack" dialog. Choose
the "zip" format. Press the "Advanced >>" button. Using the "Command line
switches" comboBox: add a new switch (for example: -r), press OK to compress
the file with that switch. Check that the file is correctly created. Close
Krusader. Check that in the ~/.config/krusaderrc file: after `Command Line
Switches=` the previously typed switch can be found.

Open Krusader again. Over a file: press Alt+Shift+P to open the "Pack" dialog
again. Press the "Advanced >>" button. Delete the previously typed switch from
the comboBox:
a) using Shift+Del when the popup list is seen;
b) or using the arrow keys and Shift+Del when the popup list is not visible.

Try to delete all the switches until the list is empty, press Shift+Del again.
Monitor the changes in the ~/.config/krusaderrc file.

Perform similar experiments having more than one switch (for example -r and
-v) in the comboBox.

Note: Those changes are designed to be tried after applying the ones
described in https://phabricator.kde.org/D28998