Give PerfTimer the ability to write only slow operations to the log

This is useful to ignore fast operations that just spam the log.

The class had to be reworked because it couldn't properly handle cases
where all of a node's children where below the threshold.

Also changed DoTimed() and RunActivity() to use PerfTimer.

OpenRA.Game/Traits/Util.cs

@@ -82,11 +82,8 @@ namespace OpenRA.Traits
 			{
 				var prev = act;
 
-				var sw = Stopwatch.StartNew();
-				act = act.Tick(self);
-				var dt = sw.Elapsed;
-				if (dt > Game.Settings.Debug.LongTickThreshold)
-					Log.Write("perf", "[{2}] Activity: {0} ({1:0.000} ms)", prev, dt.TotalMilliseconds, Game.LocalTick);
+				using (new PerfTimer("[{0}] Activity: {1}".F(Game.LocalTick, prev), (int)Game.Settings.Debug.LongTickThreshold.TotalMilliseconds))
+					act = act.Tick(self);
 
 				if (prev == act)
 					break;