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

@@ -1,5 +1,5 @@
-<ui:FancyWindow xmlns="https://spacestation14.io"
-                xmlns:ui="clr-namespace:Content.Client.UserInterface"
+<controls:FancyWindow xmlns="https://spacestation14.io"
+                xmlns:controls="clr-namespace:Content.Client.UserInterface.Controls"
                 Title="{Loc 'gravity-generator-window-title'}"
                 MinSize="270 130"
                 SetSize="360 180">
@@ -32,4 +32,4 @@
         </PanelContainer>
     </BoxContainer>
 
-</ui:FancyWindow>
+</controls:FancyWindow>

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;