ControllerInterface: Make the ID assigning code common

This makes the device ID assigning code common to all backends, by
moving it to AddDevice() instead of copy-pasting or replicating
the logic in the backends.

Also, to prepare for hotplugging, instead of relying on a name usage
count, the new ID assigning system always starts from ID 0 and tries
to assign the first ID that is not used.

Source/Core/InputCommon/ControllerInterface/XInput/XInput.cpp

@@ -171,11 +171,6 @@ std::string Device::GetName() const
   }
 }
 
-int Device::GetId() const
-{
-  return m_index;
-}
-
 std::string Device::GetSource() const
 {
   return "XInput";