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/Hands/Systems/HandVirtualItemSystem.cs

@@ -1,4 +1,5 @@
-using Content.Client.Items;
+using Content.Client.Hands.UI;
+using Content.Client.Items;
 using Content.Shared.Hands;
 using Content.Shared.Hands.Components;
 using JetBrains.Annotations;