added invoker virtual class and made timer inherit from it

src/timer.cpp

@@ -16,7 +16,7 @@ void Timer::add_time(milliseconds deltatime) {
 	accumulator += deltatime;
 	if (accumulator >= triggerinterval) {
 		accumulator -= triggerinterval;
-		callback();
+		request();
 		triggercount++;
 		if (!isinfinite && triggercount >= triggeramount){
 			isexpired=true;
@@ -24,6 +24,10 @@ void Timer::add_time(milliseconds deltatime) {
 	}
 };
 
+void Timer::request() {
+		callback();	
+}
+
 void TimerManager::add_timer(size_t triggeramount, function<bool()> myaction, std::chrono::milliseconds triggerinterval) {
 	Timer mynewtimer(triggeramount, myaction, triggerinterval);
 	timers.push_back(std::move(mynewtimer));