Fix various HiDPI, smoothing and positioning issues in previews and popups
- KGamePopupItem: Fix incorrect positioning relative to canvas
- KGamePopupItem: Use fixed offset from canvas border
- KGamePopupItem: Fix icon and text alignment in HiDPI mode
- KGameThemeSelector: Fix preview in HiDPI mode
- KgThemeProvider: Smooth previews
(See commit messages for details.)