[KHistoryComboBox] Add method to set an icon provider

Summary: An icon provider can be any function that takes a QString and returns a QIcon. This shall replace the setPixmapProvider to allow defining icons by name.

Test Plan: Tested with KDirSelectDialog and a patched plasma-integration

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12310

Differential Revision: https://phabricator.kde.org/D25700