Properly wait for app thread exit on Haiku

Properly wait for app thread exit on Haiku

* src/haiku_support.cc (MessageReceived): Handle
QUIT_APPLICATION.
(start_running_application): Clean up code a little.
(wait_for_exit_of_app_thread): New function.
(BApplication_setup): Add atexit handler to clean up app thread.
(be_app_quit): Delete function.
* src/haikuterm.c (haiku_delete_terminal): Un-implement
function.
* src/haikuterm.h: Update prototypes.