Engine and Editor subsystems

- Dynamic subsystems for fixed module startup wrt subsystem initialization
- Python Gettter exposed for both eg: import_subsystem = unreal.get_editor_subsystem(unreal.ImportSubsystem)
- Import Subsystem with python exposed import events

#rb Matt.Kuhlenschmidt, Rex.Hill
#codereview Matt.Kuhlenschmidt, Rex.Hill

[CL 4725117 by Chris Gagnon in Dev-Editor branch]

Engine/Source/Editor/EditorSubsystem/EditorSubsystem.Build.cs

@@ -0,0 +1,17 @@
+// Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.
+
+using UnrealBuildTool;
+
+public class EditorSubsystem : ModuleRules
+{
+    public EditorSubsystem(ReadOnlyTargetRules Target)
+         : base(Target)
+    {
+        PrivateDependencyModuleNames.AddRange(
+            new string[] {
+                "Core",
+                "CoreUObject",
+                "Engine",
+            });
+    }
+}

Engine/Source/Editor/EditorSubsystem/Private/EditorSubsystem.cpp

@@ -0,0 +1,9 @@
+// Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.
+
+#include "EditorSubsystem.h"
+
+UEditorSubsystem::UEditorSubsystem()
+	: UDynamicSubsystem()
+{
+
+}

Engine/Source/Editor/EditorSubsystem/Private/EditorSubsystemModule.cpp

@@ -0,0 +1,15 @@
+// Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.
+
+#include "EditorSubsystemModule.h"
+#include "Modules/ModuleManager.h"
+
+IMPLEMENT_MODULE(FEditorSubsystemModule, EditorSubsystem);
+
+void FEditorSubsystemModule::StartupModule()
+{
+
+}
+
+void FEditorSubsystemModule::ShutdownModule()
+{
+}

Engine/Source/Editor/EditorSubsystem/Public/EditorSubsystem.h

@@ -0,0 +1,28 @@
+// Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.
+
+#pragma once
+
+#include "Subsystems/Subsystem.h"
+
+#include "EditorSubsystem.generated.h"
+
+/**
+ * UEditorSubsystem
+ * Base class for auto instanced and initialized systems that share the lifetime of the Editor
+ *
+ * UEditorSubsystems are dynamic and will be initialized when the module is loaded if necessary.
+ * This means that after StartupModule() is called on the module containing a subsystem,
+ * the subsystem collection with instantiate and initialize the subsystem automatically.
+ * If the subsystem collection is created post module load then the instances will be created at
+ * collection initialization time.
+ */
+
+UCLASS(Abstract)
+class EDITORSUBSYSTEM_API UEditorSubsystem : public UDynamicSubsystem
+{
+	GENERATED_BODY()
+
+public:
+	UEditorSubsystem();
+};
+

Engine/Source/Editor/EditorSubsystem/Public/EditorSubsystemModule.h

@@ -0,0 +1,16 @@
+// Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.
+
+#pragma once
+
+#include "CoreMinimal.h"
+#include "Modules/ModuleInterface.h"
+
+class EDITORSUBSYSTEM_API FEditorSubsystemModule : public IModuleInterface
+{
+public:
+	// IModuleInterface
+	virtual void StartupModule() override;
+	virtual void ShutdownModule() override;
+
+private:
+};