Fix crash while mapping pads

Sometimes, ES crashes while mapping buttons of a new pad. This is due to the lack of synchronization mechanism between InputStack, it's internal threads, and the GUI (initial caller).

Debouncing in InputStack is now performed using timestamp instead of a threaded timer. The remaining timer has been replaced with a sync'ed timer to safely interact with the GUI. The debouncing has been moved right after the first event push so that it's now working fine with Sony pads.