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/Core/HW/GCKeyboardEmu.cpp

@@ -86,6 +86,7 @@ std::string GCKeyboard::GetName() const
 
 void GCKeyboard::GetInput(KeyboardStatus* const kb)
 {
+  auto lock = ControllerEmu::GetStateLock();
   m_keys0x->GetState(&kb->key0x, keys0_bitmasks);
   m_keys1x->GetState(&kb->key1x, keys1_bitmasks);
   m_keys2x->GetState(&kb->key2x, keys2_bitmasks);