either libVLC 2.0.5 changed the semantics of their callbacks (they can be
invoked from the main thread, i.e. from the VLC method resulting in the event);
or GTK+ 2.24 silently changed the type of the GDK mutex from recursive to simple.

in either case to avoid GDK mutex deadlocks we must check whether the VLC
callback's thread already holds the lock or owns the main context

 * arbitrary limit (1024 bytes) per line - reading lines of arbritrary length is difficult and will be seldomly used
 * introduced is_newline() helper function

 * also made gtk_experiment_transcript_free_formats() an internal function
 * declare static functions in gtk-experiment-transcript-formats.c

default interactive format attributes may be NULL (i.e. don't change these attributes) but pango/gdk free functions cannot handle NULL pointers gracefully
 * only results in assertions now since windows are explicitly destroyed (the info window was not destroyed at all previously)

removes some constraints on resizing the info window containing the widget
(however other widgets have minimum sizes preventing the transcript area being squashed together)

also required stripping trailing newlines when processing "format" files since "format" files only worked because the newline was part of the "format" expression and ignored