ControllerInterface: Add RemoveDevice()

This adds RemoveDevice() to ControllerInterface, fixes ExpressionParser
and some other code to support device removals without crashing,
and adds an IsValid() method to Device, to prepare for hotplugging.

Source/Core/InputCommon/ControllerEmu.h

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <cmath>
 #include <memory>
+#include <mutex>
 #include <string>
 #include <vector>
 
@@ -444,6 +445,12 @@ public:
 
   void UpdateReferences(ControllerInterface& devi);
 
+  // This returns a lock that should be held before calling State() on any control
+  // references and GetState(), by extension. This prevents a race condition
+  // which happens while handling a hotplug event because a control reference's State()
+  // could be called before we have finished updating the reference.
+  static std::unique_lock<std::recursive_mutex> GetStateLock();
+
   std::vector<std::unique_ptr<ControlGroup>> groups;
 
   ciface::Core::DeviceQualifier default_device;