Use weston_compositor_exit instead of wl_display_terminate

We have a weston_compositor API for exiting the compositor and letting the frontend prepare for exit, and it also allows setting the process exit code. This MR changes most places that were using wl_display_terminate() to call the higher level API.

The main reason for this is to forward failure to run weston-desktop-shell as Weston's process exit code, so that tests where it fails will actually be marked as failed.

Unifying the use of the API is another reason and a tiny but necessary step towards making weston_compositor opaque.

I did not touch the remaining wl_display_terminate() in main.c as they didn't have weston_compositor readily available.