Perform an action the next time Emacs is idle for SECS seconds.
The action is to call FUNCTION with arguments ARGS.
SECS may be an integer, a floating point number, or the internal
time format returned by, e.g., `current-idle-time'.
If Emacs is currently idle, and has been idle for N seconds (N < SECS),
then it will call FUNCTION in SECS - N seconds from now. Using
SECS <= N is not recommended if this function is invoked from an idle
timer, because FUNCTION will then be called immediately.
If REPEAT is non-nil, do the action each time Emacs has been idle for
exactly SECS seconds (that is, only once for each time Emacs becomes idle).
This function returns a timer object which you can use in `cancel-timer'.