Ghost UI to XAML and minor cleanup (#4637)

* Remove unused class, clean up XAML

* Move GhostGui.cs into UI folder and namespace

* GhostTargetWindow to seperate file

* GhostTargetWindow to XAML

* Center request button

* Improve UI, localisation

Content.Client/Ghost/GhostSystem.cs

@@ -1,4 +1,5 @@
-using Content.Client.HUD;
+using Content.Client.Ghost.UI;
+using Content.Client.HUD;
 using Content.Shared.Ghost;
 using JetBrains.Annotations;
 using Robust.Client.GameObjects;