allow programLogo property to be a QIcon, too

Summary: rational: QIcon allows for better passing of HiDPI aware icons to the dialog

Test Plan:
make && make test

tried out to use an QIcon in KTextEditor, seems to work and it looks "crisp"

Reviewers: apol

Reviewed By: apol

Subscribers: dhaumann, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26903