Allow dbus processing in qt-base to enable color-picking via portal

qt-base uses dbus to query the desktop portal if color-picking is supported, without explicitly waiting for the result. kcolorchooser creates its QColorDialog before the response was processed and therefore color picking via the portal is presumed to be unavailable.

Give the eventloop the opportunity to process the event and only afterwards create the QColorDialog.

See https://bugreports.qt.io/browse/QTBUG-120957 Closes https://bugs.kde.org/show_bug.cgi?id=479406