KgThemeProvider: Smooth previews

Some KDE Games allow to switch between different themes. The theme
selector shows previews, based on pixmaps which are scaled down to fit
within the available UI space. By default `Qt::FastTransformation` is
used within `KgThemeSelector`, however this can result in subpar image
quality, even for non-HiDPI use cases.

Changing the scaling method to `Qt::SmoothTransformation` improves the
look of the previews. Since `generatePreview` is not exactly a hot code
path, performance implications of the change should not be an issue.

Note: The similar class `KGameThemeSelector` is already using
`Qt::SmoothTransformation` for its previews.

Test Plan:

Open KPatience, "Settings -> Change Appearance -> Game Theme" now shows
less blocky previews.