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.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