mediastream: Rename an api to avoid name collisions

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 (closed)