Expose Control Expression variables to mappings UI

-add a way to reset their value (from the mappings UI)
-fix "memory leak" where they would never be cleaned,
one would be created every time you wrote a character after a "$"
-fix ability to create variables with an empty string by just writing "$" (+added error for it)
-Add $ operator to the UI operators list, to expose this functionality even more

Source/Core/DolphinQt/Config/Mapping/IOWindow.h

@@ -111,6 +111,7 @@ private:
   // Shared actions
   QPushButton* m_select_button;
   QComboBox* m_operators_combo;
+  QComboBox* m_variables_combo;
 
   // Input actions
   QPushButton* m_detect_button;