kcms/wallpaper: add some keywords for discoverability

Shubham Arorarequested to merge
work/shubham/wallpaper-keywords into master

Couldn't find the kcm when searching for background in krunner when trying out 6.6 wallpaper proposals.

Added a few more keywords as well from the default list of plugins as well as some users can download from store.kde.org

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