qml Windows native style is broken on dark mode: switch to Fusion

qml Windows native style is broken on dark mode: switch to Fusion

we will temporarily be using Fusion generic desktop style

Close https://github.com/nextcloud/desktop/issues/7215

see upstream Qt bug
https://bugreports.qt.io/browse/QTBUG-113469?focusedId=724929&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-724929

they apparently do not plan to fix Windows qml style to support dark
mode on Windows 11

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>