GCPadEmu: only connected if default device connected

This lets Dolphin know if a configured GameCube Controller should actually
be treated as connected or not.

Talked to @JMC47 a bit about this last night. My use-case is that all of
my controllers are the same hardware (Xbox One controllers) so share the
same configuration (modulo device number). Treating them all as always
connected isn't a problem for most games, but in some (Smash Bros.) it
forces me to go find a keyboard/mouse and unconfigure any controllers
that I don't actually have connected. Hotplugging devices (works on macOS,
at least) + this patch remove my need to ever touch the Controller Config
dialog while in a game.

This patch makes the following changes:

- A new `BooleanSetting` in `GCPadEmu` called "Always Connected", which
  defaults to false.
- `ControllerEmu` tracks whether the default device is connected on every
  call to `UpdateReferences()`.
- `GCPadEmu.GetStatus()` now sets err bit to `PAD_ERR_NO_CONTROLLER` if
  the default device isn't connected.
- `SIDevice_GCController` handles `PAD_ERR_NO_CONTROLLER` by imitating the
  behaviour of `SIDevice_Null` (as far as I can tell, this is the only use
  of the error bit from `GCPadStatus`).

I wanted to add an OSD message akin to the ones when Wiimotes get
connected/disconnected, but I haven't yet found where to put the logic.

Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp

@@ -34,6 +34,8 @@ std::unique_lock<std::recursive_mutex> EmulatedController::GetStateLock()
 void EmulatedController::UpdateReferences(const ControllerInterface& devi)
 {
   const auto lock = GetStateLock();
+  m_default_device_is_connected = devi.HasConnectedDevice(m_default_device);
+
   for (auto& ctrlGroup : groups)
   {
     for (auto& control : ctrlGroup->controls)
@@ -48,6 +50,11 @@ void EmulatedController::UpdateReferences(const ControllerInterface& devi)
   }
 }
 
+bool EmulatedController::IsDefaultDeviceConnected() const
+{
+  return m_default_device_is_connected;
+}
+
 const ciface::Core::DeviceQualifier& EmulatedController::GetDefaultDevice() const
 {
   return m_default_device;

Source/Core/InputCommon/ControllerEmu/ControllerEmu.h

@@ -33,6 +33,7 @@ public:
   virtual void LoadConfig(IniFile::Section* sec, const std::string& base = "");
   virtual void SaveConfig(IniFile::Section* sec, const std::string& base = "");
 
+  bool IsDefaultDeviceConnected() const;
   const ciface::Core::DeviceQualifier& GetDefaultDevice() const;
   void SetDefaultDevice(const std::string& device);
   void SetDefaultDevice(ciface::Core::DeviceQualifier devq);
@@ -49,5 +50,6 @@ public:
 
 private:
   ciface::Core::DeviceQualifier m_default_device;
+  bool m_default_device_is_connected{false};
 };
 }  // namespace ControllerEmu

Source/Core/InputCommon/ControllerInterface/Device.cpp

@@ -199,5 +199,11 @@ Device::Output* DeviceContainer::FindOutput(const std::string& name, const Devic
 {
   return def_dev->FindOutput(name);
 }
+
+bool DeviceContainer::HasConnectedDevice(const DeviceQualifier& qualifier) const
+{
+  const auto device = FindDevice(qualifier);
+  return device != nullptr && device->IsValid();
+}
 }
 }

Source/Core/InputCommon/ControllerInterface/Device.h

@@ -158,6 +158,8 @@ public:
   std::string GetDefaultDeviceString() const;
   std::shared_ptr<Device> FindDevice(const DeviceQualifier& devq) const;
 
+  bool HasConnectedDevice(const DeviceQualifier& qualifier) const;
+
 protected:
   mutable std::mutex m_devices_mutex;
   std::vector<std::shared_ptr<Device>> m_devices;