Port away from KF5 deprecated API: use setWindowIcon

Test Plan:
Restarted kwalletd5, made it open a wallet (via kwalletmanager),
the dialog that pops belongs to kwalletd5 and has a nice icon.

Reviewers: vkrause, ngraham

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24968