Prefer QIcon::pixmap(QWindow*, ...) overload

Summary:
It takes into account the dpi of the screen we're rendering to.
Other overloads assume the window is nullptr and will use the primary screen's dpi which can change almost randomly.

Test Plan: Icons still look fine even if I drag windows from a screen to another.

Reviewers: Plasma, Frameworks, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29102