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/Gravity/UI/GravityGeneratorWindow.xaml.cs

@@ -1,6 +1,6 @@
 using System;
 using Content.Client.Message;
-using Content.Client.UserInterface;
+using Content.Client.UserInterface.Controls;
 using Content.Shared.Gravity;
 using Robust.Client.AutoGenerated;
 using Robust.Client.GameObjects;