Using timers to target unspecified problems

This is a part of Advanced Gtk+ Sequencer out of ags_async_queue.h

define AGS_ASYNC_QUEUE_CLOCK_ID CLOCK_REALTIME

define AGS_ASYNC_QUEUE_SIGNAL_HIGH SIGRTMAX

define AGS_ASYNC_QUEUE_SIGNAL_LOW SIGRTMIN

Instead queueing nanosleep there will be timers used.