[AppChooser] Don't use KBuildSycocaProgressDialog to rebuild sycoca

Nicolas Fellarequested to merge
work/nico/sycoca-exec into master

It uses a nested event loop internally, which causes issues with the QML-based UI

Instead call the ksycoca exe manually

BUG: 484739

SENTRY: XDG-DESKTOP-PORTAL-KDE-K

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