refactors adminmenu a timid amount (#5095)

Co-authored-by: Paul <ritter.paul1+git@googlemail.com>
Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Client/Administration/AdminSystem.cs

@@ -0,0 +1,67 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Content.Shared.Administration;
+using Content.Shared.Administration.Events;
+using Robust.Client.Player;
+using Robust.Shared.GameObjects;
+using Robust.Shared.IoC;
+using Robust.Shared.Log;
+using Robust.Shared.Network;
+using Robust.Shared.Players;
+
+namespace Content.Client.Administration
+{
+    public partial class AdminSystem : EntitySystem
+    {
+        [Dependency] private readonly IPlayerManager _playerManager = default!;
+
+        public event Action<List<PlayerInfo>>? PlayerListChanged;
+
+        private Dictionary<NetUserId, PlayerInfo>? _playerList;
+        public IReadOnlyList<PlayerInfo> PlayerList
+        {
+            get
+            {
+                if (_playerList != null) return _playerList.Values.ToList();
+
+                return new List<PlayerInfo>();
+            }
+        }
+
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            InitializeOverlay();
+            InitializeMenu();
+            SubscribeNetworkEvent<FullPlayerListEvent>(OnPlayerListChanged);
+            SubscribeNetworkEvent<PlayerInfoChangedEvent>(OnPlayerInfoChanged);
+            SubscribeNetworkEvent<PlayerInfoRemovalMessage>(OnPlayerInfoRemoval);
+        }
+
+        private void OnPlayerInfoRemoval(PlayerInfoRemovalMessage ev)
+        {
+            if (_playerList == null) _playerList = new();
+
+            _playerList.Remove(ev.NetUserId);
+            PlayerListChanged?.Invoke(_playerList.Values.ToList());
+        }
+
+        private void OnPlayerInfoChanged(PlayerInfoChangedEvent ev)
+        {
+            if(ev.PlayerInfo == null) return;
+
+            if (_playerList == null) _playerList = new();
+
+            _playerList[ev.PlayerInfo.SessionId] = ev.PlayerInfo;
+            PlayerListChanged?.Invoke(_playerList.Values.ToList());
+        }
+
+        private void OnPlayerListChanged(FullPlayerListEvent msg)
+        {
+            _playerList = msg.PlayersInfo.ToDictionary(x => x.SessionId, x => x);
+            PlayerListChanged?.Invoke(msg.PlayersInfo);
+        }
+    }
+}