plugins: Find upgrade background images from new well-known locations

Rather than shipping one in-tree and requiring distributions to patch gnome-software downstream to customise it.

See individual commit messages and, particularly, the new doc/vendor-customisation.md file for details.

This doesn’t change the overall appearance of the upgrade banner, just the logic used to choose the background image.

Signed-off-by: Philip Withnall pwithnall@endlessos.org