deal with plasma translucency by forcing static color wallpapers

it's become clear that the translucency randomly makes needles not pass for
no better reason than wallpaper elements bleeding through due to
translucency. as we do not really want to disable all effects and
translucency has no actual setting to disable it we'll instead force a
static wallpaper color for both plasma and the lock screen.

this should give fairly stable core needles for plasma. with only a few
actually asserting the actual wallpapers. the latter will need redoing
whenever the wallpaper changes, but by and large this should make needles
fairly consistent across versions