Make decay work like other controls

Updating the control should trigger a re-render.
Move the callback to callbacks.c.
Remove deprecated g_thread_init call

Also add a necessary header to avoid a compiler warning.
Remove unnecessary dependency on gthread

According to
https://developer.gnome.org/glib/2.39/glib-Deprecated-Thread-APIs.html#g-thread-init
it is only necessary when using g_thread_init, which is deprecated.
Add JACK output support

Closes issue #1