Handle GUI input while inside popup dialog on Haiku

Handle GUI input while inside popup dialog on Haiku

* src/haiku_support.cc (alert_popup_value): New variable.
(be_alert_thread_entry): New function.
(BAlert_go): Complete rewrite that allows async input to be
handled while the popup is active.
* src/haiku_support.h: Update prototypes.
* src/haikumenu.c (haiku_dialog_show, haiku_popup_dialog): Stop
blocking input and pass required callbacks to `BAlert_go'.
* src/haikuterm.c (haiku_term_init): Set interrupt input mode to
t.