Avoid re-creating title version string every frame (#2117)

* Avoid re-creating title version string every frame

* Marked title version string that it has been changed by Dalamud by
  appending a newline payload with a string expression embedded within,
  which the game will ignore (effectively `<br(!Dalamud!)>`.)
* Added `PluginManager.LoadedPluginCount` which will count the number of
  loaded plugin without making a copy of the plugin list.
* Made TitleScreenMenuWindow.OnVersionStringDraw` update the title
  version text addon only if number of loaded plugin changes or the text
  is missing the custom suffix from the first point.

* Use OriginalTextPointer to determine if SetText(ROS) has been called

Dalamud/Plugin/Internal/PluginManager.cs

@@ -162,6 +162,27 @@ internal class PluginManager : IInternalDisposableService
     /// </summary>
     public static int DalamudApiLevel { get; private set; }
 
+    /// <summary>
+    /// Gets the number of loaded plugins.
+    /// </summary>
+    public int LoadedPluginCount
+    {
+        get
+        {
+            var res = 0;
+            lock (this.pluginListLock)
+            {
+                foreach (var p in this.installedPluginsList)
+                {
+                    if (p.State == PluginState.Loaded)
+                        res++;
+                }
+            }
+
+            return res;
+        }
+    }
+
     /// <summary>
     /// Gets a copy of the list of all loaded plugins.
     /// </summary>