New API: Timer Callbacks

[michaelrsweet]

It would be useful to have a way to make the system object periodically call an application function. This same functionality could also be used to schedule periodic tasks that are currently hardcoded in the papplSystemRun function.

bool (*pappl_timer_cb_t)(pappl_system_t *system, void *timer_data);

bool papplSystemAddTimerCallback(pappl_system_t *system, time_t start, double interval, pappl_timer_cb_t timer_cb, void *timer_data);

The timer callback returns true to repeat or false to stop. If "start_time" is non-0, the first timer callback will occur at the specified time, otherwise it will happen in "interval" seconds.

[michaelrsweet]

[master 774bbed] Add timer API (Issue #208)