SDL: Re-add frame step hotkey

Needs porting to Qt at some point.

src/duckstation-sdl/sdl_host_interface.cpp

@@ -351,6 +351,8 @@ bool SDLHostInterface::Initialize()
     return false;
   }
 
+  RegisterHotkeys();
+
   ImGui::NewFrame();
 
   // process events to pick up controllers before updating input map
@@ -1334,6 +1336,16 @@ void SDLHostInterface::ClearImGuiFocus()
   ImGui::SetWindowFocus(nullptr);
 }
 
+void SDLHostInterface::RegisterHotkeys()
+{
+  RegisterHotkey(StaticString("General"), StaticString("FrameStep"), StaticString("Frame Step"), [this](bool pressed) {
+    if (!pressed)
+    {
+      DoFrameStep();
+    }
+  });
+}
+
 void SDLHostInterface::DoStartDisc()
 {
   Assert(!m_system);

src/duckstation-sdl/sdl_host_interface.h

@@ -72,6 +72,7 @@ private:
   void DestroyDisplay();
   void CreateImGuiContext();
   void UpdateFramebufferScale();
+  void RegisterHotkeys();
 
   /// Executes a callback later, after the UI has finished rendering. Needed to boot while rendering ImGui.
   void RunLater(std::function<void()> callback);