input_common: Eliminate most global state

Abstracts most of the input mechanisms under an InputSubsystem class
that is managed by the frontends, eliminating any static constructors
and destructors. This gets rid of global accessor functions and also
allows the frontends to have a more fine-grained control over the
lifecycle of the input subsystem.

This also makes it explicit which interfaces rely on the input subsystem
instead of making it opaque in the interface functions. All that remains
to migrate over is the factories, which can be done in a separate
change.

src/yuzu/main.h

@@ -40,12 +40,20 @@ namespace Core::Frontend {
 struct SoftwareKeyboardParameters;
 } // namespace Core::Frontend
 
+namespace DiscordRPC {
+class DiscordInterface;
+}
+
 namespace FileSys {
 class ContentProvider;
 class ManualContentProvider;
 class VfsFilesystem;
 } // namespace FileSys
 
+namespace InputCommon {
+class InputSubsystem;
+}
+
 enum class EmulatedDirectoryTarget {
     NAND,
     SDMC,
@@ -62,10 +70,6 @@ enum class ReinitializeKeyBehavior {
     Warning,
 };
 
-namespace DiscordRPC {
-class DiscordInterface;
-}
-
 class GMainWindow : public QMainWindow {
     Q_OBJECT
 
@@ -86,8 +90,6 @@ public:
     GMainWindow();
     ~GMainWindow() override;
 
-    std::unique_ptr<DiscordRPC::DiscordInterface> discord_rpc;
-
     bool DropAction(QDropEvent* event);
     void AcceptDropEvent(QDropEvent* event);
 
@@ -255,6 +257,9 @@ private:
 
     Ui::MainWindow ui;
 
+    std::unique_ptr<DiscordRPC::DiscordInterface> discord_rpc;
+    std::unique_ptr<InputCommon::InputSubsystem> input_subsystem;
+
     GRenderWindow* render_window;
     GameList* game_list;
     LoadingScreen* loading_screen;