Fix Pidgin debug window with threaded logging

I was seeing several broken `GtkTextIter`s which eventually caused a crash at random. This was due to TLS support, which writes its logs from a non-main thread. And there are probably other cases of threaded logging in GLib internals as well.

This change avoids calling GTK functions (which are not thread safe) from the log handler.

Testing Done:
Connected to a local server with TLS, that previously caused guaranteed crashes.

Reviewed at https://reviews.imfreedom.org/r/1292/