wayland: convert most manual or QObject memory management to std::unique_ptr

In a few places it was not obvious who is supposed to own the created objects or in which order they need to be destroyed. That's not completely fixed yet, but Display is now at least more explicitly destroyed before interface extensions.

There also were some places where manual management could be very easily replaced 1:1 with std::unique_ptr, and there were a lot of CLEANUP macros in the autotests that did what std::unique_ptr is for.