wallpapers/image: add support for custom accent color
The custom color is specified by "X-KDE-PlasmaImageWallpaper-AccentColor" in the metadata.
Depends on plasma-desktop!1128 (merged) and https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/587
Test package: Altai.tar.gz
Edited by Fushan Wen