Use org.kde.desktop on Windows

This makes sure the app uses our Breeze style, and only with org.kde.desktop or org.kde.breeze + KIconThemes do icons show up on Windows.

See:

• https://develop.kde.org/docs/getting-started/kirigami/introduction-getting_started/#craft
• https://invent.kde.org/frameworks/qqc2-desktop-style
• https://invent.kde.org/plasma/qqc2-breeze-style