Summary:
SVGs do not honor the "Transparent background" config option, and simply have no
background at all. This patch ensures this option is applied to SVGs as well as
raster images.

Unlike `RasterImageView` that uses a `QPixmap` buffer (`mCurrentBuffer`) for drawing
the background and the image, SVGs are rendered directly on top of `SvgImageView`
(`QGraphicsWidget`). Therefore we just paint the background in `SvgImageView::paint`
which happens before the SVG is rendered.

We move the checkboard texture code to `AbstractImageView` now that it's used by
both subclasses `RasterImageView` and `SvgImageView`.

This patch also configures kconf_update due to the moving of the above enum.
Since I didn't want to litter `/app` with files, I've moved the update files to
`/kconf_update`.

Fixes T8125

Before:
{F5764298}

After:
{F5764299}
{F5764300}

Test Plan:
  - Open an SVG with solid color config option
  - Open and SVG with checkboard background con...