Move/Rename stuff (#11209)

-     Change namespace, and folder of FancyWindow to Content.Client.UserInterface.Controls
-     Change xaml reference from ui to controls in some places
-     Change ClientAlertsSystem from internal to public
-     Change namespace, and folder of HighDivider to Content.Client.UserInterface.Controls

Content.Client/UserInterface/Controls/HighDivider.cs

@@ -0,0 +1,14 @@
+using Content.Client.Stylesheets;
+using Robust.Client.UserInterface;
+using Robust.Client.UserInterface.Controls;
+
+namespace Content.Client.UserInterface.Controls
+{
+    public sealed class HighDivider : Control
+    {
+        public HighDivider()
+        {
+            Children.Add(new PanelContainer {StyleClasses = {StyleBase.ClassHighDivider}});
+        }
+    }
+}