Fix reading wallpaper config from LNF and plasma themes

Fabian Vogtrequested to merge
work/fvogt/wallpaperfix into master

This reverts commit e3b2e688.

The default in the plugin config unfortunately takes precendence over the LNF and plasma theme supplied configs.

CC @fusionfuture

I'll have a quick look if I can come up with a proper fix for kcm_lockscreen.

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