window-actor/wayland: Remove subsurface actors on dispose

From the main commit:

Destroying the window actor also destroys all its children. Subsurfaces
however may get reused. If the client did not unparent them before
the window actor got destroyed, they will be left without actor
which results in a crash.

Unparent all actors of subsurfaces on dispose to avoid that.

And some cleanups.

Closes #1892 (closed)