macOS: widget UI flashes when overriding color scheme during UI construction

We implemented the widget gallery example to override the color scheme to Dark in the WidgetGallery constructor. When launching that example via the bundle (from finder or open gallery.app), then the UI shows up with a white background at first, and then changes to dark. This also happens when overriding the style, so it's not a QMacStyle specific issue.

When launching the example's executable directly (i.e. ./gallery.app/Contents/MacOS/gallery), then the problem doesn't reproduce.

The palette is correct in all cases (light palette before the WidgetGallery constructor, dark afterwards), and we don't construct the NSWindow until we show the widget.

As a workaround we now override the color scheme right away, but that's just a stop-gap solution.