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/Administration/QuickDialogSystem.cs

@@ -1,5 +1,5 @@
 using System.Linq;
-using Content.Client.UserInterface;
+using Content.Client.UserInterface.Controls;
 using Content.Shared.Administration;
 using Robust.Client.UserInterface;
 using Robust.Client.UserInterface.Controls;