IdleMonitor: new class to replace GlobalEvents::IDLE

Use MaskMonitor to eliminate duplicate code.

src/Idle.cxx

@@ -27,25 +27,12 @@
 #include "Main.hxx"
 #include "Instance.hxx"
 
-#include <atomic>
-
 #include <assert.h>
 
-static std::atomic_uint idle_flags;
-
 void
 idle_add(unsigned flags)
 {
 	assert(flags != 0);
 
-	unsigned old_flags = idle_flags.fetch_or(flags);
-
-	if ((old_flags & flags) != flags)
-		instance->global_events.Emit(GlobalEvents::IDLE);
-}
-
-unsigned
-idle_get(void)
-{
-	return idle_flags.exchange(0);
+	instance->EmitIdle(flags);
 }