Add central function scheduler (#609)

* Add central function scheduler

* Avoid unnecessary copies

* Lint

* Prevent more copies, store pointers

* Add never update_interval

esphome/core/application.h

@@ -6,6 +6,7 @@
 #include "esphome/core/preferences.h"
 #include "esphome/core/component.h"
 #include "esphome/core/helpers.h"
+#include "esphome/core/scheduler.h"
 
 #ifdef USE_BINARY_SENSOR
 #include "esphome/components/binary_sensor/binary_sensor.h"
@@ -197,6 +198,8 @@ class Application {
   }
 #endif
 
+  Scheduler scheduler;
+
  protected:
   friend Component;