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/HotkeyManager.cpp

@@ -240,6 +240,7 @@ std::string HotkeyManager::GetName() const
 
 void HotkeyManager::GetInput(HotkeyStatus* const kb)
 {
+  auto lock = ControllerEmu::GetStateLock();
   for (int set = 0; set < (NUM_HOTKEYS + 31) / 32; set++)
   {
     std::vector<u32> bitmasks;