Add nag dialog regarding PLC

Source/RP0.csproj

@@ -59,6 +59,7 @@
     <Compile Include="GameplayTips.cs" />
     <Compile Include="HarmonyPatcher.cs" />
     <Compile Include="Harmony\BaseCrewAssignmentDialog.cs" />
+    <Compile Include="StartupPopup.cs" />
     <Compile Include="LoadingScreenChanger.cs" />
     <Compile Include="Maintenance\MaintenanceSettings.cs" />
     <Compile Include="Maintenance\MaintenanceHandler.cs" />

Source/StartupPopup.cs

@@ -0,0 +1,45 @@
+using System;
+using System.IO;
+using System.Reflection;
+using UnityEngine;
+
+namespace RP0InstallChecker
+{
+    [KSPAddon(KSPAddon.Startup.Instantly, true)]
+    public class StartupPopup : MonoBehaviour
+    {
+        private const string PreferenceFileName = "RP1UpgradeWarning";
+        private static string PreferenceFilePath => Path.Combine(
+            Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location),
+            "PluginData",
+            PreferenceFileName);
+
+        public void Start()
+        {
+            if (File.Exists(PreferenceFilePath)) return;
+
+
+                PopupDialog.SpawnPopupDialog(
+                    new Vector2(0, 0),
+                    new Vector2(0, 0),
+                    new MultiOptionDialog(
+                        "RP1StartupDialog",
+                        "This is the legacy version of RP-1, and is no longer supported. There is a new version of RP-1, the Programs and Launch Complexes rework, now available. It is savebreaking, and a work in progress, but if you are starting a new save it is highly recommended to switch to that version. Please uninstall RP-1 (Legacy) and install RP-1 (PLC). You can do this by uninstalling and reinstalling the RP-1 Express Install and choosing one of the non-Legacy options.",
+                        "RP-1",
+                        HighLogic.UISkin,
+                        new DialogGUIVerticalLayout(
+                            new DialogGUIButton("Don't show again", RememberPreference, true),
+                            new DialogGUIButton("Ok", () => { }, true)
+                            )
+                        ),
+                true,
+                    HighLogic.UISkin);
+        }
+
+        private static void RememberPreference()
+        {
+            // create empty file
+            File.Create(PreferenceFilePath).Close();
+        }
+    }
+}