This does the following:

• Link BreezeIcons library in AppImage build, and set fallback icon theme to Breeze if there are not any.
• Copy relevant plugins for qqc2-desktop-style into AppDir
• Set the QQuickStyle to org.kde.desktop if there is none set, in the AppImage hooks

Type: fix

Run the AppImage (preferably under non-Plasma) and verify that it is styled properly.