Fix memory management for internal Completer of the combo box editor (when extraValueAllowed=true)

Summary:

• Fix memory management
• Add a property with extraValueAllowed=true for the example app

FIXED-IN:3.1.0

Test Plan:
Try List, List 2 and Editable List properties of the kpropertyexample app.
Try e.g. data source property for elements in KEXI reports or KReportExample

Reviewers: piggz

Reviewed By: piggz

Subscribers: Kexi-Devel-list

Tags: KProperty

Differential Revision: https://phabricator.kde.org/D9940