[cpp] Sync led::delay_{on,off} properties with sysfs attributes

Fixed naming; added readers; allow chaining of writers

Author: ddemidov

Diff for: ev3dev.cpp

@@ -839,8 +839,8 @@ void led::flash(unsigned interval_ms)
     for (int i = 0; ; ++i) {
       std::this_thread::sleep_for(std::chrono::milliseconds(100));
       try {
-        set_on_delay (interval_ms);
-        set_off_delay(interval_ms);
+        set_delay_on (interval_ms);
+        set_delay_off(interval_ms);
         break;
       } catch(...) {
         if (i >= 5) throw;

Diff for: ev3dev.h

@@ -1239,8 +1239,18 @@ class led : protected device
   void off() { set_brightness(0); }
 
   void flash(unsigned interval_ms);
-  void set_on_delay (unsigned ms) { set_attr_int("delay_on",  ms); }
-  void set_off_delay(unsigned ms) { set_attr_int("delay_off", ms); }
+
+  int delay_on() const { return get_attr_int("delay_on"); }
+  auto set_delay_on (int ms) -> decltype(*this) {
+    set_attr_int("delay_on", ms);
+    return *this;
+  }
+
+  int delay_off() const { return get_attr_int("delay_off"); }
+  auto set_delay_off(unsigned ms) -> decltype(*this) {
+    set_attr_int("delay_off", ms);
+    return *this;
+  }
 
   static led red_right;
   static led red_left;