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/InputCommon/ControllerEmu/ControllerEmu.cpp

@@ -44,6 +44,8 @@ void EmulatedController::UpdateReferences(const ControllerInterface& devi)
   ciface::ExpressionParser::ControlEnvironment env(devi, GetDefaultDevice(), m_expression_vars);
 
   UpdateReferences(env);
+
+  env.CleanUnusedVariables();
 }
 
 void EmulatedController::UpdateReferences(ciface::ExpressionParser::ControlEnvironment& env)
@@ -75,7 +77,27 @@ void EmulatedController::UpdateSingleControlReference(const ControllerInterface&
                                                       ControlReference* ref)
 {
   ciface::ExpressionParser::ControlEnvironment env(devi, GetDefaultDevice(), m_expression_vars);
+
   ref->UpdateReference(env);
+
+  env.CleanUnusedVariables();
+}
+
+const ciface::ExpressionParser::ControlEnvironment::VariableContainer&
+EmulatedController::GetExpressionVariables() const
+{
+  return m_expression_vars;
+}
+
+void EmulatedController::ResetExpressionVariables()
+{
+  for (auto& var : m_expression_vars)
+  {
+    if (var.second)
+    {
+      *var.second = 0;
+    }
+  }
 }
 
 bool EmulatedController::IsDefaultDeviceConnected() const