sddm and sddm idle split into 5.15

also adding sddm-2, this is a divergent 5.15 wallpaper but with 5.16 blur
changes, so it's different but strongly related to 5.15. keeping this
to not regress current installations

also drop idle vs idle-unstable. I haven't checked the history but it makes
no sense. we have TYPE tags, they should be used to select a given
needle for a specific type

also unlimit sddm from unstable. no use only having it tested there TBH
only gives a headache once user/testing roll over to new wallpaper and
we assert this kinda elsewhere anyway