Some bindings can't handle the coexistence of
GtkMediaStream:prepared and gtk_media_stream_prepared.
Help them out by renaming the function to
gtk_media_stream_set_prepared, and rename
gtk_media_stream_unprepared as well, to match.
The existing entry points still exist, deprecated.
Update all internal uses.
Fixes: #4023
Visual Studio 2013 is just shy of being sufficiently C99-compliant to
build GTK master, as it did not support snprintf() in its CRT
implementation.
Use g_snprintf() to cover for this.
