Core: Add initial System class

Introduces the system class that will eventually contain all relevant
system state, as opposed to everything being distributed all over the
place as global variables.

Throughout the codebase we have code that from its interface-view, does
not actually require its dependencies to be described in the interface,
and we routinely run into issues with initialization where we sometimes
make use of a facility before it's been initialized, which leads to
annoying to debug cases, because the reader needs to run through the
codebase and see what order things get initialized in, and how they're
being used. This is particularly a frequent issue in the video code.

Further, we also have a lot of code that makes use of file-scope
variables (many of which are non-trivial), which must all be default
initialized before the application can actually enter main(). While this
may not be a huge issue in itself, some of these are allocating, which
means that the application may need to use memory that it otherwise
wouldn't need to (e.g. when a game isn't running, this excess memory is
being used).

Being able to wrap all these subsystems into objects would be nicer,
since they can be constructed when they're actually needed. Them being
objects also means we can better express dependencies on subsystems as
types directly in the interface, making them explicit to the reader
instead of a change randomly blowing up, said reader inspecting it, and finding
out that something needed to be initialized beforehand. With the global
turned into a function parameter, the dependency is explicit and they
know just by reading it, that the given subsystem needs to be in a valid
state before calling the function.

For a prior example of an emulator that has moved to this model, see
yuzu, which has been migrated off of global variables all over the place
and replaced with a system instance (which has now reached the stage,
where the singleton can be removed).

Source/Core/Core/CMakeLists.txt

@@ -41,6 +41,8 @@ add_library(core
   SyncIdentifier.h
   SysConf.cpp
   SysConf.h
+  System.cpp
+  System.h
   TitleDatabase.cpp
   TitleDatabase.h
   WiiRoot.cpp

Source/Core/Core/Core.vcxproj

@@ -371,6 +371,7 @@
     <ClCompile Include="PowerPC\SignatureDB\SignatureDB.cpp" />
     <ClCompile Include="State.cpp" />
     <ClCompile Include="SysConf.cpp" />
+    <ClCompile Include="System.cpp" />
     <ClCompile Include="TitleDatabase.cpp" />
     <ClCompile Include="WiiRoot.cpp" />
     <ClCompile Include="WiiUtils.cpp" />
@@ -678,6 +679,7 @@
     <ClInclude Include="State.h" />
     <ClInclude Include="SyncIdentifier.h" />
     <ClInclude Include="SysConf.h" />
+    <ClInclude Include="System.h" />
     <ClInclude Include="Titles.h" />
     <ClInclude Include="TitleDatabase.h" />
     <ClInclude Include="WiiRoot.h" />

Source/Core/Core/Core.vcxproj.filters

@@ -187,6 +187,7 @@
     <ClCompile Include="PatchEngine.cpp" />
     <ClCompile Include="State.cpp" />
     <ClCompile Include="SysConf.cpp" />
+    <ClCompile Include="System.cpp" />
     <ClCompile Include="TitleDatabase.cpp" />
     <ClCompile Include="WiiRoot.cpp" />
     <ClCompile Include="WiiUtils.cpp" />
@@ -1022,6 +1023,7 @@
     <ClInclude Include="PatchEngine.h" />
     <ClInclude Include="State.h" />
     <ClInclude Include="SysConf.h" />
+    <ClInclude Include="System.h" />
     <ClInclude Include="Titles.h" />
     <ClInclude Include="TitleDatabase.h" />
     <ClInclude Include="WiiRoot.h" />

Source/Core/Core/System.cpp

@@ -0,0 +1,18 @@
+// Copyright 2020 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include "Core/System.h"
+
+namespace Core
+{
+struct System::Impl
+{
+};
+
+System::System() : m_impl{std::make_unique<Impl>()}
+{
+}
+
+System::~System() = default;
+}  // namespace Core

Source/Core/Core/System.h

@@ -0,0 +1,36 @@
+// Copyright 2020 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+
+namespace Core
+{
+// Central class that encapsulates the running system.
+class System
+{
+public:
+  ~System();
+
+  System(const System&) = delete;
+  System& operator=(const System&) = delete;
+
+  System(System&&) = delete;
+  System& operator=(System&&) = delete;
+
+  // Intermediate instance accessor until global state is eliminated.
+  static System& GetInstance()
+  {
+    static System instance;
+    return instance;
+  }
+
+private:
+  System();
+
+  struct Impl;
+  std::unique_ptr<Impl> m_impl;
+};
+}  // namespace Core