#152739 fix needs additional method in KCompletionBox

[Germain Garand]

Hi,

It seems fixing the showstopper #152739 requires adding a virtual method in 
KCompletionBox, as KCompletionBox's popup() method is not practically 
re-implementable to suit our needs.

Please see the patch attached to said bug report and the comments #4/#5.
I'd like to commit that patch on Monday afternoon if you agree.

Thanks,
Germain