Allow small timeout intervals in IdleInterface

A threshold exists to stop users flooding the server for no reason.

However, there is a usecase for small timeouts.

rsibreak has a "please relax for 20 seconds" interface. Here it makes
perfect sense to know if a user is active in small increments. The plan
is to start a 1s timer and wait for that. Then we wait locally for 20s
without a resume event.

src/wayland/idle_interface.cpp

@@ -144,8 +144,8 @@ void IdleTimeoutInterface::setup(quint32 timeout)
     }
     timer = new QTimer(this);
     timer->setSingleShot(true);
-    // less than 5 sec is not idle by definition
-    timer->setInterval(qMax(timeout, 5000u));
+    // less than 500 msec is not idle by definition
+    timer->setInterval(qMax(timeout, 500u));
     QObject::connect(timer, &QTimer::timeout, this,
         [this] {
             send_idle();