kcms/decoration: Fix double click behavior over the thumbnail

KDecoration.Decoration eats all the events for the sake of keeping an illusion of buttons being like the real ones. I didn't find any sane way to completely turn off event filtering on that level, so instead let's fix this MouseArea, which is a workaround, but a decent one.

AFAIK this is the last KCM in Plasma to properly support "double-click to save" behavior. There might be others though: watch out for event filters inside thumbnail.

(cherry picked from commit 63ff64b0)