Show application window icon on AboutPage

Summary:
With this it is not nessecary to KAboutData::setProgramLogo. If the
QApplication::setWindowIcon is set, then the AboutPage will
automatically use that icon.

Test Plan:
Tested with Discover, with only setWindowIcon, only setProgramLogo and
both setWindowIcon and setProgramLogo

Reviewers: mart, apol, Kirigami, hein

Reviewed By: mart, Kirigami, hein

Subscribers: bcooksley, sitter, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D23241