app: make backtrace processed in the thread where error happens.

Slight back step from commit 34fe992f. I don't keep track anymore of
the number of errors inside GimpCriticalDialog. The problem is that GTK+
calls must happen in the main thread, and errors in another thread will
be delayed into the main thread through gdk_threads_add_idle_full().
This makes any backtrace generated as a consequence of a threaded error
useless (in particular any error happening in GEGL since we always
process these as multi-threaded, whether they are or not).
Instead I now keep track of the number of errors in gui-message.c, which
still allows to reset the counters when the unique debug dialog is
closed. Therefore I can now generate backtraces conditionally to the
error counters inside the problematic thread (and right when the error
happened), without any GTK+ call.
This finally makes GEGL backtraces useful in the debug dialog! :-)